by
Norman R. King
Professor of Geology
University of Southern Indiana
Evansville, IN 47712
INTRODUCTION
GEOLOGIC BACKGROUND: THE UPS AND
DOWNS OF SEA LEVEL
ROCK UNITS EXPOSED AT I-170 (in
order as in the text)
Bandera
Formation
Varicolored Claystone
Mulberry Member
Conglomerate and Limestone
Altamont Formation
Amoret Limestone Member
Lake Neosho Shale Member
Calcareous Shale
Blocky Claystone
Phosphatic Shale
Bioclastic Shale
Lake Neosho Shale Member Summary
Worland Limestone Member
Hard Gray Limestone
Yellow Weathering (Clayey) Limestone
Rooted Limestone
Worland Limestone Member Summary
Nowata
Shale
Summary
of Depositional History
REFERENCES
MEASURED
SECTION
This report on ongoing research describes and interprets
Middle Pennsylvanian (upper
Desmoinesian) strata exposed in road cuts for I-170 in St. Louis, Missouri (photo),
where the highway skirts the east side of Lambert International Airport. This
is one of
the best exposures of the Bandera and Altamont Formations in Missouri (photo).
Most
Pennsylvanian strata were removed by erosion from southern and east-central
Missouri,
but the Bandera and Altamont have been preserved here due to a local downwarp
called the St. Louis basin. The nearest Pennsylvanian exposures in the Midcontinent
region are in the outcrop belt that trends from northeastern Oklahoma across
southeastern Kansas, northward through the Kansas City area, and into northwestern
Missouri and southern Iowa. This area is referred to as the "western outcrop
belt" in this
report (Text-Figure 1).
The I-170 site is readily accessible, although heavy,
high-speed traffic makes it
dangerous to cross the highway (photo).
Visitors should make judicious use of
offramps and onramps to drive "around the block" to get from one side
of the highway to
the other, rather than trying to outleg three lanes of traffic approaching at
55 miles per
hour or faster! Visitors should also take special care in working below the
limestone
caprock because rock falls have occurred with regularity, and some portions
of the
highway cut are clearly unstable (photo).
Portions of the Bandera and Altamont Formations in St.
Louis are very fossiliferous
(photo). In the 1930's, J. Brookes Knight
published several papers on Pennsylvanian
molluscan faunas from the St. Louis area. More recently, amateurs have collected
thousands of additional fossil specimens, including several previously undescribed
taxa.
Some of these fossils are illustrated at the www.lakeneosho.org website. In
part, this
study was intended to provide a stratigraphic and sedimentologic context for
those fossil
collections.
On-line publication allows this to be a "living"
document. It can be improved and
broadened as people suggest viable alternative explanations for features and
relationships described and pictured here. Readers may also relate information
about
similar phenomena that provide additional insight into the stratigraphic units
and their
origins. New data from the site can be integrated into the existing report at
any time.
This report will therefore never become outdated, and it will evolve as information
accumulates. To encourage a public dialog, reader comments and questions about
technical matters may be posted in the "Comments and Discussion" section.
On-line publication cannot replace conventional publication outlets. Both modes
have
their own objectives, capabilities, and values. For example, the numerous high-
resolution, full color photographs in this report document the rocks and interpretations
at a level that is impossible for traditional media.
Some of the figure captions include descriptive detail
and interpretations not found in
the text, so the best way to study this paper is to print the complete text
so it will be
available off-line while viewing the photographs and reading the captions on-line.
The
tone and subject treatment are appropriate for advanced amateurs and college
geology students. Nevertheless, none of the technical descriptions and conclusions
have been published previously, so professional geologists may also find this
presentation useful. The priority, however, was to make the information accessible
to
a broader audience than those who typically read specialized technical journals.
Readers having limited technical background may find it useful to have a geological
glossary handy for looking up unfamiliar terms. Many such glossaries are available
on-line.
GEOLOGIC BACKGROUND: THE UPS AND DOWNS OF SEA LEVEL
The Bandera Formation and Altamont Formation were deposited
during the middle of
the Pennsylvanian Period, about 305 million years ago. This was near the end
of a time
interval known as the Desmoinesian Age, named for rock exposures along the Des
Moines River in Iowa. In the Midcontinent region, including Missouri, upper
Desmoinesian rock formations are assigned to the Marmaton Group (Text-Figure
2a).
Rocks of the same age are present elsewhere in North America and beyond, but
the
lithologies and formational nomenclature are different.
There are both similarities and differences between
North America in the Pennsylvanian
Period and the North America of today. By the Pennsylvanian, for example, the
central
part of the continent--the Midcontinent region--had already been a stable lowland
for
hundreds of millions of years. During the Pennsylvanian, however, sea level
was often
higher than today, so much of the Midcontinent was frequently under water. At
such
times marine sedimentary rocks were deposited in places that are now dry land.
At
other times sea level was hundreds of feet lower and the Midcontinent was high
and
dry. The sedimentary rocks record these conditions.
Mountains existed in the Appalachian region during the
Pennsylvanian, but these
mountains were much higher than today's Appalachians. The Rocky Mountains had
not yet formed. Instead, several small uplifts separated by basins existed in
the
western states and in Oklahoma and adjacent parts of Texas. A series of highlands
extended across Arkansas and Texas along the southern edge of the continent.
Closer
to St. Louis, there was a minor highland region in southern Missouri called
the Ozark
dome, near the present-day St. Francis Mountains (Text-Figure
1).
Pennsylvanian floras and faunas were quite different
from those of today. Amphibians
still dominated terrestrial vertebrate faunas, since the first reptiles had
only appeared
near the beginning of the period. Brachiopods, crinoids, bryozoans, and platy
calcareous algae dominated the shallow, clear water of marine (salt water) settings.
Molluscs such as clams and snails were more common in muddy waters near shore.
Fern trees and the primitive scale trees dominated coastal lowlands, while conifers
had
recently become important in dryer settings. Flowering plants were not to develop
for
another 150 million years.
Climatic fluctuations caused significant glacial advances
and retreats during the
Pennsylvanian Period. The magnitude of Pennsylvanian climatic swings may have
approached those during the so-called "Ice Age" of the past two million
years. Of
course, glaciers advance when conditions are cold, and retreat when conditions
are
warm. Sea level falls when glaciers advance because a significant amount of
the
Earth's water becomes locked in glacial ice on land. When the climate becomes
warmer, sea level rises because melting glaciers send huge volumes of meltwater
back
to the sea. This is why sea level varied so much during the Pennsylvanian, alternately
flooding and then exposing the Midcontinent lowlands. Rotational and orbital
parameters of the Earth appear to cause climatic cyclicity, but further discussion
is
beyond the scope of this report. Heckel (1994) documented the climatic control
of
Pennsylvanian marine incursions, and a web search on "Milankovitch cycles"
will
provide more information on the astronomical variations that affect climate.
When sea level rises, shorelines move landward as the
sea "transgresses" across
formerly dry land. When sea level falls, the sea "regresses" back
toward the open
ocean. In the Midcontinent region of North America, each Pennsylvanian cycle
of sea
level rise and fall produced a familiar sequence of rocks showing the transition
from
terrestrial to marine conditions, and back again. At the beginning of each cycle,
rising
sea level caused formation of coastal swamps and marshes as river gradients
decreased and run-off water ponded on the nearly flat coastal plains. The terrain
would
have looked much like swampy land today in the Great Dismal Swamp on the Virginia
North Carolina border, or the Okeefenokee Swamp on the Georgia-Florida border.
Peat
forms in marshes and swamps, and peat that becomes deeply buried under younger
deposits eventually turns to coal. Typically, shale was deposited in near-shore
water
that inundated the coastal-plain marshes as sea level rose. The shale is overlain,
in
turn, by limestone that formed when the water became clearer--when the shoreline
had moved so far away that little or no clay reached this location. The limestone
is
composed largely of calcareous "skeletons" (shells and the like) of
marine
invertebrates and algae that lived on the sea floor. These skeletal particles
are
embedded in fine-grained carbonate sediment (called "micrite"--a contraction
for
"microcystalline calcite") that precipitated in the warm, clear seawater.
The
environmental setting during limestone deposition was generally similar to portions
of The Bahamas and the Florida Keys today.
During many cycles the sea became so deep that not enough
light for photosynthesis
reached the sea floor. Carbonate-producing algae and microbial communities ceased
growing, so limestone deposition came to an end. Heckel (1986) estimated that
water
depth in the Midcontinent region may have reached, or even exceeded, 100 meters
(330 feet). That would have put bottom water below the influence of wave agitation,
and stagnation would occur in the absence of oceanic currents to keep water
circulating.
Bottom-dwelling (benthic) invertebrates would diminish in variety and number
as the
oxygen content of bottom water declined, and many deep-water shales contain
fossils
of only swimming and floating organisms. Fine clay particles and organic matter
from
dead swimmers and floaters would slowly accumulate on the sea floor, producing
dark-
colored shale rich in organic matter and containing phosphate nodules and laminae.
Eventually the climate began to cool. More and more
snowfall in interior and polar
regions became locked in glacial ice instead of returning to the sea, causing
sea level to
drop. As the sea became shallower, light could reach the sea floor again. Benthic
algae
returned, helping to replenish the supply of oxygen in the bottom water. A diverse
benthic fauna proliferated, and limestone was deposited again. Eventually the
water was so shallow that currents from wave motion and tides impinged on the
sea
floor, tumbling and fragmenting shells and algal plates. Signs of deposition
at or near
sea level are often found near the tops of these regressive limestones, including
broken
pieces of already-lithified carbonate sediment that were ripped up and redeposited
by
storm waves, laminated sediment formed by shallow-water algal mats, and root
structures made by plants that colonized subaerially exposed stretches.
The entire Midcontinent region became dry land during
the lowest stands of sea level.
As the sea withdrew, streams brought clay, silt, and sand to be deposited in
shallow
water near shore. Ultimately, deltas, beaches, and marshes prograded seaward,
building vast plains analogous to coastal regions of Texas and Louisiana today.
Subaerially exposed deposits were subjected to weathering and erosion. Infiltrating
rainwater, rooting by plants, and churning by burrowing animals promoted chemical
and
physical changes that led to soil ("paleosol") formation. Rivers incised
valleys into the
terrain, but sediments filled these during the next rise of the sea.
ROCK UNITS EXPOSED AT I-170
Pennsylvanian rock strata in Missouri include limestones
and shales of marine origin
alternating with non-marine mudrocks and minor rock types such as sandstone
and
coal. A quick look at any Pennsylvanian outcrop is generally inadequate to identify
the
formation exposed there because the rock types and the sequence in which they
occur
are similar in successive formations. Normally, a specific rock unit can be
identified only
through fossils. Geologists have long been dependent upon the appearance and
disappearance of fossil species through geologic time to correlate beds at one
outcrop
with a "master" sequence of beds that has been pieced together from
data collected at
hundreds of sites over many decades of work. Microscopic conodonts and fusulinid
foraminifers have been used extensively for correlating Pennsylvanian marine
strata
throughout North America and beyond. Conodonts have received most of the attention
in recent years. They evolved relatively quickly during the Pennsylvanian, and
the
conodont animals were swimmers, accounting for the very widespread distribution
of
individual species. These attributes make conodonts among the best "index
fossils" for
Pennsylvanian beds. Pennsylvanian marine shales often contain large numbers
of
conodonts, and the shales can be disaggregated easily and sieved to recover
the
fossils. Limestones also contain conodonts, but they are fewer and the carbonate
is
difficult to process.
The conodont species recently named Swadelina neoshoensis
by Lambert and others
(2003) is abundant in all of the marine shales at the I-170 outcrop. This taxon
ranges
from the underlying Pawnee Formation to the Lenapah Limestone in the western
outcrop belt (the Pawnee data is new to this report; Heckel [1999] reported
on the
remainder of the range, referring to Swadelina neoshoensis as Idiognathodus
sp. 5
of Swade [1986]). Other conodonts recovered abundantly from the shales at I-170
include Idiognathodus delicatus, Idioprioniodus sp., Adetognathus
sp., and several
species of Neognathodus. See Ritter and others (2002) for a thorough
review of
Pennsylvanian conodont biostratigraphy and illustrations of the significant
conodont
index species.
Bandera Formation
The Bandera Formation includes strata
lying between the Pawnee Formation below and
the Altamont Formation above (Text-Figure
2a). Neither the Pawnee nor lowermost
Bandera are exposed at I-170. Brightly colored claystone at the base of the
exposure is
in the middle part of the Bandera. It is overlain abruptly by dark colored mudrocks
with
coal and limestone of the Mulberry Member. Lenticular algal limestone above
the
Mulberry may correlate with the Farlington Limestone Bed in the western outcrop
belt.
The upper part of the Bandera at I-170 includes mainly conglomerate composed
of
clasts up to boulder size (larger than 256 mm) of claystone and limestone embedded
in
shaly matrix. Lenses of algal fragment, lithoclastic, and "shell hash"
limestone are
common in the conglomerate, showing deposition in marine waters. The Mulberry
and
Farlington are the only formally named subdivisions within the Bandera.
Varicolored Claystone
The lowest beds exposed at I-170 include mottled, brightly colored claystone,
referred
to here informally as the "varicolored claystone" unit (photo).
Colors include, in
approximate order of abundance, various shades of gray, maroon, orange, yellow,
brown, and purple, along with all manner of gradation between them (photo).
The
"background" matrix color is gray in the upper part (photo)
and maroon in the lower
part (photo). Prominent patches and vertical
streaks of dark yellowish orange, along
with greenish gray to bluish gray indicate the former positions of roots. Some
of the
root structures in the uppermost part of the unit are at least partially replaced
by solid
iron oxide, and are termed "rhizocretions" (photo).
This unit has the characteristics of
an ancient soil zone, or "paleosol." It formed during a significant
fall in sea level when
the region was exposed to weathering and was colonized by terrestrial vegetation.
Bright colors generally indicate the presence of iron
compounds. Anhydrous iron oxide,
or hematite, produce red and maroon. Hydrous iron oxides, often referred to
collectively
as "limonite," produce yellow, orange, and brown. Organic matter and
reduced iron
minerals such as pyrite produce the greenish tints as well as drab colors such
as dark
gray and olive gray.
Reddish (anhydrous) iron oxides form in well-drained
soils with deep water table that
allows full exposure to oxygen. Yellow, orange, and brown (hydrous) iron oxides
indicate
moister conditions, perhaps due to wetter climate or slower drainage. Marine
submergence
or a perennially high water table with swampy conditions saturate the soil with
oxygen-poor
water, preserving organic detritus and causing reduction of iron compounds.
Dark
colored sediment or soil results. Although presence of iron oxides is often
thought to
indicate warm, humid conditions in the environment, topography also plays a
role.
Dissected terrain is better drained than flat terrain, so the degree of oxidation
may vary
from hillsides to flat terrain under identical climatic regimes. Oxidation during
recent
weathering has probably also contributed to the spectacular coloration of this
unit.
The varied colors are thoroughly intermixed, revealing
a complex history of oxidation
and reduction, as well as disruption of original sedimentary stratification
during soil
formation. Conditions in the soil changed from oxidizing to reducing, and back
again,
and rooting by plants mixed the different zones--shallow material descended
into root
holes as woody tissues decayed, bringing soil material of different colors into
close
contact (photo).
The uppermost part of the varicolored claystone at I-170
is dark gray where fresh
(it becomes lighter upon weathering), showing that the final influence on the
soil was
reducing rather than oxidizing. In fact, carbonaceous sediment in the overlying
Mulberry Member lies only a fraction of a foot above the top of the claystone
along
the northern portion of the outcrop. This shows that marshy (reducing!) conditions
occupied the formerly dry land as it became waterlogged and eventually submerged
during early stages of the ensuing marine transgression.
The claystone is thoroughly slickensided. "Slickensides"
are the shiny, slippery surfaces
of fractures (photo) that result from parallel
orientation of clay minerals. They form in
three different ways. Tectonic slickensides form where opposite sides of faults
slide past
each other, reorienting clay minerals concentrated along the fault surfaces.
Pedogenic
slickensides originate from repeated expansion and contraction of soils caused
by
alternate wetting and drying. Units of soil, called "peds" (photo)
slide back and forth
against each other, reorienting tiny clay flakes on either side of the surfaces.
Slickensides also form along "microfaults" formed during compaction
of clay-rich
sedimentary rocks (photo), especially next
to relatively non-compactable objects such
as nodules, or next to irregularities at contacts with less compactable rocks
such as
sandstone and limestone. Compactional slickensides are present in most mudrocks
and
locally in clayey limestone of the Worland Limestone at I-170. Minor pedogenic
slickensides may also be present in the varicolored claystone and in a locally
developed
paleosol between the Mulberry Member of the Bandera Formation and Lake Neosho
Shale Member of the Altamont Formation.
Intensive and prolonged action by soil-forming processes
completely obliterated original
features of the sediment. The original sediment could have been shale, siltstone,
or
even limestone. Clay minerals typically form during chemical weathering, so
the clay-
rich nature of the paleosol is probably secondary. No fossils have been found
in this
interval. Mature paleosols are usually unfossiliferous because any fossils that
may have
been present in unaltered sediment were destroyed during soil formation.
Mulberry Member
The varicolored claystone is overlain by a complex interval of shale, claystone,
limestone, and coal that only rarely exceeds one foot in thickness along the
I-170
outcrop. Coal at this stratigraphic level elsewhere in the Midcontinent region
has been
named the Mulberry Coal Bed. Spore and pollen fossils recovered from the Mulberry
elsewhere indicate it correlates with the Danville Coal in the Illinois basin
(Peppers,
1996), an important commercial coal. In the St. Louis area, however, the coal
is thin and
impure. At I-170 the Mulberry consists of anastomosing coaly laminae interbedded
with
calcareous to carbonaceous shale and claystone (photo)
with scattered lenses and
clasts of limestone and phosphate nodules. Since the coaly material occurs as
separate
laminae and very thin beds rather than a single bed, and since coal forms only
a minor
portion of the unit, it is referred to in this report simply as the Mulberry
Member of the
Bandera Formation. It includes the entire interval above the varicolored claystone
and
below the overlying conglomeratic zone.
The contact between the varicolored claystone and Mulberry
Member is slightly
undulating (photo), probably due in
part to scouring and channeling on the surface of
transgression by waves and tidal currents as the sea advanced across the terrain.
Locally, claystone pebbles in shaly matrix rest on the contact (photo);
lag gravels of
this sort are common above ancient erosion surfaces. A conspicuous thin layer
of
reddish (hematitic) clay marks the base of the Mulberry along much of the I-170
exposure (photo), although the sediment
that hosted the iron oxide precipitate was
scoured out in a few places. The iron was probably formed late in the history
of the
sediment. Similar mineralized zones or surfaces are common at horizons that
separate
strata formed during different depositional cycles. Minerals become concentrated
where
differences in permeability impede the vertical flow of ground water. Limestone
in lenses
at the base of the Mulberry has been bleached to light shades of gray, yellow,
and
biege, so textures and grain types are obscure (photo).
These limestones apparently
got that way because they were immersed for long periods of time in chemically
reactive
water that became perched above this permeability barrier.
Coaly material is present at different levels in the
Mulberry Member. It marks the base
of the member in some paces, and elsewhere it forms several thin interbeds with
shale
and claystone, or forms a conspicuous layer above a basal zone of shale and
claystone
containing limestone lenses and clasts (photo).
Locally, however, the shaly interval in
the lower Mulberry is overlain directly (no coal) by richly fossiliferous, calcareous
claystone to shale that is mottled olive gray to dark gray where fresh (northern
part of
the exposure; photo), but is greenish gray
where weathered (southern part of the
exposure; photo). The fossils tend to be
inconspicuous because they are poorly
preserved or are microscopic. This gray claystone is absent along much of the
outcrop,
apparently because it was removed during a brief episode of erosion prior to
deposition
of overlying strata. Mulberry fossils include conodonts, agglutinated forams,
ostracodes,
sponge spicules, gastropods, pelecypods, scaphopods, brachiopods, branching
bryozoans, and vertebrate material (scales, spines, and teeth).
Phosphate nodules are abundant in the Mulberry. These
are small, light brown to gray,
lumpy masses that resemble pebbles (photo).
Cracking them open often reveals a
fish scale, tooth, or spine, or an inarticulate brachiopod shell, all of which
are rich in
phosphate. In fact, these phosphatic skeletal materials apparently served as
nuclei for
precipitation of the phosphate. X-ray analysis shows the phosphate to be fluorapatite
[Ca5(PO4)3.F].
The origin of phosphate in strata deposited in epicontinental seas is
controversial. Phosphate nodules in Pennsylvanian shales have been interpreted
as
signifying relatively deep water (Kidder, 1985), apparently having formed within
poorly
oxygenated sediment in areas having upwelling currents in the overlying water
column.
Paleogeographic reconstructions for the Midcontinent region by Heckel (1980)
indicate
that upwelling was likely in this part of the Pennsylvanian sea when it reached
its
greatest depths (100 meters or somewhat more). Many dark-colored Pennsylvanian
shales appear to have been deposited in water of the required depth, but shale
in the
Mulberry Member seems to be the product of a shallower setting. The dark color
confirms poor oxygenation, but this is more likely a result of deposition near
swamps or
marshes along the shoreline than in deep water. Despite abundant organic matter,
the
water was at least periodically oxygenated well enough to support a bottom fauna
of
moderate diversity. Close association of marine fossils, limestone, and coal
confirm a
near-shore depositional environment (photo).
The coaly material probably formed in marshes analogous
to modern coastal swamps,
with a moderately diverse fauna living near-by in the shallow subtidal zone.
Shells and
shell fragments accumulated offshore in tidal channels or in mounds heaped up
by
currents, eventually forming the lenticular limestone beds. Some of these shelly
limestone beds were broken into pieces during storms. Their sharp edges were
rounded off as they were tumbled about, and then they were redeposited as clasts.
Fine-grained sediment exposed along the edges of incised tidal channels or along
the
surf zone at the shoreline was also eroded, forming claystone clasts. If those
shaly beds
also contained interbeds of limestone, both clast types may have had their origin
there.
The phosphate nodules might also have been reworked. Scouring at the base of
the
Mulberry resulted from strong tidal currents or storm waves.
Like today, slight fluctuations in sea level occurred
constantly during the Pennsylvanian.
Rises caused occasional drowning of coastal swamps, accounting for the interbedding
of coal and shelly layers in the Mulberry. Conversely, slight drops in sea level
would
result in emergence. Rooting by Pennsylvanian plants, consistent with deposition
in
very shallow coastal waters or even on dry land, mixed different layers of sediment
to
produce the mottling visible in the upper part of the claystone along the northern
part of
the exposure. As the transgression continued, olive gray to dark gray calcareous
and
fossiliferous claystone was deposited when the coastal terrain became completely
submerged. Persistent strong currents continued to erode previously deposited
sediments, so that any part (or all) of the Mulberry is missing at various locations
along
the outcrop.
As the transgression proceeded, lenticular beds of algal
limestone were deposited in
clear water far from shore. These lenses, which may correlate with the Farlington
Limestone Bed in the western outcrop belt, lie at or near the base of the overlying
conglomerate and limestone unit (see below). The upper contact of the Mulberry
is
placed where the strata become dominated by either limestone or conglomerate.
Conglomerate and Limestone
The Mulberry Member at I-170 is overlain by matrix-supported conglomerate
consisting
of clasts of claystone and limestone embedded in shaly matrix (photo).
This interval is
referred to here as the "conglomerate and limestone" unit of the Bandera
Formation.
Lenticular beds of in situ limestone (formed in place, rather than as
transported clasts)
are present throughout the interval. It also contains scattered phosphate nodules,
some
of which lie on edge and were clearly reworked from previously deposited shaly
beds.
Lenticular beds of micritic algal limestone at the base
of the conglomerate and limestone
unit (photo) may correlate with the Farlington
Limestone in the western outcrop belt, but
index fossils range too long to distinguish the Farlington from the Amoret Limestone.
Platy calcareous algae form the bulk of the skeletal material in these lenses.
The algae,
often referred to as "phylloid algae," produced leafy fronds around
which either calcite
or aragonite precipitated. Some fronds encrusted the sea floor, and others grew
in
upright positions. As photosynthesizers, the algae required clear water that
allowed
bright sunlight to reach the bottom. Like modern algae, they could probably
live at depths
greater than 100 meters, but probably grew luxuriantly in water no deeper than
a few
tens of meters. Several modern algae also produce calcareous skeletons (photo),
generating prodigious amounts of carbonate sediment in modern oceans. Clarity
of
water has always been the critical ecological factor, so algal limestones in
this unit
indicate deposition in clear, shallow water beyond the area where muddy sediment
had
settled out. Thin beds and lenses of in situ lithoclastic limestone (photo)
are also
common in the lower part of the conglomerate and limestone unit, and there are
numerous thin lenticular in situ beds of coquinoid ("shell hash")
limestone in the upper
part (photo) of the unit. These limestone
types are discussed below.
Rounded limestone clasts derived from previously deposited
beds are locally very
conspicuous in the conglomerate and limestone unit. These were eroded from older
limestone beds, in part by waves breaking against the ancient shoreline. Others
may have come from lenses of limited extent that were exhumed from the shallow
sea
floor by storm waves. Most of the original limestones were highly fossiliferous,
containing scattered whole and fragmented fossils embedded in micritic to finely
fragmented skeletal matrix. Some of the limestone clasts include algal lamination
(photo). Fossils of platy calcareous algae
and invertebrates (brachiopods and crinoids,
for example) indicate the original limestones were deposited under fully marine
conditions in clear, shallow water. A drop in sea level allowed wave action
and tidal
currents to scour the sea floor and erode the previously deposited strata. Some
of the
limestones were rooted (photo), showing
that for a while sea level was so low that
terrestrial plants colonized the terrain. Most of the clasts became rounded
due to
prolonged buffeting by waves and currents. Others are quite jagged, indicating
they
were redeposited quickly. The larger clasts, some more than one foot in diameter
(photo), could not have been transported
far. These clasts served as a hard substrate
for chitons to graze (they feed on the soft green algae that coat rocks). Chiton
shell
material is found in shelly limestones of the conglomerate and limestone unit
but
nowhere else at the I-170 exposure.
More numerous and mostly smaller, angular to rounded
clasts of claystone (photo)
were deposited with the limestone clasts. This suggests that the original limestone
beds
were interbedded with claystone, and that claystone dominated the sequence.
The
softer claystone produced generally smaller clasts than the more resistant limestone,
but some boulder-sized claystone clasts are present (photo).
These clasts may have
been produced by the collapse of a steep, wave-cut (and possibly undercut) slope
along the shoreline. The clasts did not move far before being engulfed in clay-rich
material derived from less catastrophic erosional mechanisms. Claystone clasts
that
remained exposed to the surf disintegrated, further contibuting to the clayey
matrix of
the conglomerate.
Presence of lime mud (micrite) along with clay in the
matrix of lithoclastic limestones
(photo) shows that the seawater was highly
saturated with calcium carbonate. High-
energy conditions normally would have swept away both lime mud and clay, except
that
apparently so much clay was being liberated by erosion of clay-rich beds and
clasts,
and so much lime mud precipitating near-by, that large volumes of fine-grained
material were "dumped" here faster than they could be swept away.
Moreover, strong
currents may have been intermittent, especially if generated only during major
storms.
A modern setting that is presently producing lithoclastic limestone is shown
in the
attached photograph (photo). Little or no
sand or other grain sizes are present in the
conglomerate and limestone unit at I-170 because the terrain that supplied the
sediment consisted only of clayey materials and the limestone beds.
In the upper part of the conglomerate are thin undulating
lenses of algal-fragment
micritic limestone (photo). These limestones
must have formed in place, because
such thin layers would have broken into small pieces if eroded and transported.
Although at least some of the claystone clasts and conglomeratic matrix may
have come from a terrestrial settng by gravity-induced transport, these materials
must have come to rest in a marine setting where platy calcareous algae lived.
Near the middle of the outcrop on the west side of the
highway, a prominent bed of
relatively fine-grained lithoclastic limestone lines the bottom of a channel-shaped
scour
feature (photo). This was probably a tidal
channel analogous to those along tide-
dominated shorelines in the modern Bahamas.
Bioclastic ("shell hash") limestones are common
in the upper part of the conglomerate
and limestone unit (photo). One variety
consists of thinly interlaminated shelly lime-
stone lenses and shale (photo). The matrix
becomes less limy upward, and there is
a discontinuous layer of coquinoid limestone consisting of closely packed fossils
embedded in non-calcareous clay matrix at the top of the unit (photo).
Most fossils
in the "shell hash" beds (both micritic and clayey) are more or less
complete shells of
brachiopods, gastropods, pelecypods, and calcareous algal plates, with a scattering
of other kinds of shells including scaphopods, nautiloid cephalopods, chiton
valves,
and trilobites. The well-preserved fossils in these shelly layers suggest the
organisms
lived offshore in a less energetic setting than was normal for the conglomerate
and
limestone facies. Shells were washed onto a beach or piled into offshore shell
banks
during storms to form the shell hash layers. The accompanying photo
shows one
possible modern analog.
The shaly matrix in the conglomerate and limestone unit
is mottled with shades of
drab red and greenish gray; the reddish hues dominate. It is darkest where freshest—
where it has not been intensely weathered in modern times. This includes places
where no karstic passages have been dissolved through the Worland during recent
weathering, so no acidic, oxygen-rich water has been able to infiltrate into
the shale.
Where karstic solution passages do penetrate the Worland (photo),
the shaly matrix is
reddish due to oxidation of iron compounds. In addition, the limestone lenses
and clasts
in the conglomerate and limestone unit have been replaced by a porous variety
of
microcrystalline silica often called "tripoli" (photo).
Most solution passages in the
Worland are filled with Quaternary sediment washed in from above. A few of the
voids, however, were filled from below with underlying material, including Lake
Neosho
Shale and shaly material from the conglomerate and limestone unit (photo).
The
precise mechanism of this injection is uncertain, but it was most likely associated
with
instability of the modern outcrop.
The conglomerate and limestone unit varies from zero
to nearly four feet in thickness
along the I-170 outcrop. It thickens southward on the west side of the interstate,
where it
contains the greatest amount of limestone. It pinches out to nothing toward
the north,
where most of the clasts are claystone and there are no limestone interbeds.
Spotty
occurrence and thickness variations suggest its deposition was affected by small-scale
variations in topography and degree of exposure to wave action.
High-energy conditions that produced the conglomerate
and limestone unit reflect a
sea level drop that caused erosion of previously deposited beds of shale, claystone,
and
limestone. Huge clasts of the older sediment apparently fell into the surf zone
as
exposures along the shoreline were undercut by breaking waves. The eroded material
may then have spread out and flowed into deeper water in debris flows. Just
like today,
storm generated waves and saturation of the terrain by heavy rainfall would
have been
especially effective in causing shoreline slope failures. Although major erosional
events
were probably sporadic, the terrain was easily eroded, resulting in production
of large
volumes of sediment that choked the near-shore zone. Lenticular limestone beds
consisting of skeletal debris embedded in limy or clayey matrix were deposited
in the
same area during the intervening quieter times.
Altamont Formation
In its type area along the southern part of the Kansas-Missouri border,
the Altamont
Formation includes three members: Amoret Limestone, Lake Neosho Shale, and
Worland Limestone (ascending order). The Amoret interval is quite variable throughout
the Midcontinent region. Cline and Greene (1950) published numerous measured
sections of the upper part of the Marmaton Group in Missouri, including several
sections
where they could not identify the Amoret with certainty. Therefore they were
also
uncertain where to draw the contact between the Bandera Formation and Altamont
Formation at those locations. Their conclusions were followed in The Stratigraphic
Succession in Missouri (Howe and Koenig, editors, 1961) and in the update to
that by
Thompson (1995). The name "Farlington Limestone" had not been used
prior to 1999,
and in years before that some workers may have confused the Farlington with
the
Amoret (for example, possibly in sections IX through XII published by Cline
and
Greene).
In contrast to the Amoret, the Lake Neosho Shale and Worland Limestone are
widespread and distinctive, and they can be recognized reliably throughout the
Midcontinent region.
Amoret Limestone Member
Cline and Greene’s work (1950) showed the considerable variability of the
Amoret
Limestone in Missouri. The variability displayed at I-170 is typical. At its
type locality in
Bates County, Missouri the Amoret consists of almost four feet of micritic and
nodular
limestone. At several other Missouri localities there is no limestone at the
stratigraphic
level where the Amoret normally occurs. Cline and Greene also reported localities
where corroded clasts of algal limestone are present at the Amoret level rather
than a
continuous limestone bed (these conglomeratic zones were also noted by Heckel,
1999). This reveals a complex history similar to that for the conglomerate and
limestone
unit in the Bandera at I-170, involving deposition of limestone followed by
erosion and
redeposition of the eroded limestone clasts.
Cline and Greene (1950) believed that the Amoret was
deposited in a transgressive-
regressive cycle that preceded the Altamont depositional cycle, an opinion echoed
more recently by Heckel (1999). In this scenario, the Amoret was eroded during
a
subsequent fall of sea level, supplying the "corroded limestone clasts"
found locally in a
transgressive zone at the base of the Lake Neosho Shale. Thus, the limestone
that
used to be present in this area was assumed to correlate with the Amoret
in the type
area, and the zone of corroded limestone clasts would be younger than that.
In fact,
however, the source of the limestone clasts in this interval is unknown. Although
some
or all of them may indeed be from a pre-existing Amoret Limestone, others (or
all of
them) may be from Farlington-equivalent strata or even older limestones. If
the latter is
true, then the zone of corroded limestone clasts might be the same age as the
Amoret,
and there is no need to hypothesize an additional cycle of deposition and erosion.
The thin, discontinuous layers of clay-rich shelly limestone
totaling only a few inches
directly beneath the Lake Neosho (at the top of the conglomerate and limestone
unit) at
I-170 may be the transgressive deposits of the Altamont depositional cycle.
According
to original stratigraphic definitions (see Cline and Greene, 1950, p. 18), that
means
these shelly layers are the Amoret Limestone Member of the Altamont Formation.
However, the shelly layers are quite different from the four feet of micritic
and nodular
limestone at the type locality of the Amoret. And, if the hypothesis [Cline
and Greene
(1950) and Heckel (1999)] is correct that the zone of corroded limestone clasts
below
the Lake Neosho found here and there in Missouri represents a later depositional
cycle
than the Amoret at its type locality, then the Amoret would be older than the
clay-rich
shelly layer at I-170. Therefore, any transgressive deposits associated with
the Lake
Neosho at I-170 could not be labeled as Amoret. There are clearly some uncertainties
here. Resolving these issues of stratigraphic nomenclature and depositional
cycle
identification is beyond the scope of this report.
Unfortunately, biostratigraphic criteria for identifying
the Amoret Limestone and for
distinguishing it from the Farlington Limestone at I-170 have not been established.
In
the western outcrop belt, the Farlington to Lake Neosho (including the Amoret)
interval
contains the conodont Swadelina neoshoensis. Heckel (1999) has distinguished
Farlington from Lake Neosho strata in the western outcrop belt by relative abundances
of Swadelina and another conodont genus, Neognathodus. Swadelina
dominates in the
Farlington, and Neognathodus dominates in the Lake Neosho. Throughout
this interval
the species are the same, however, and the observed differences in abundance
may
simply reflect differing ecological conditions. Moreover, relative abundances
for the
Amoret itself have not been specified. Physical correlation is also problematical.
All
limestones at I-170 from the Mulberry to the top of the Lake Neosho are discontinuous,
even at this single exposure. Therefore, in absence of either physical continuity
or
diagnostic index fossils, none of the I-170 limestones can be definitively identified
as
an exact equivalent to either the Farlington or Amoret in their type areas.
Problems
such as these relating to recognition and correlation of the Amoret, identification
of
depositional cycles, and finding reliable index fossils show why stratigraphers
still have
work to do.
Lake Neosho Shale Member
In contrast to the Amoret, the Lake Neosho Shale Member and overlying Worland
Limestone Member of the Altamont Formation extend throughout the Midcontinent
region. The Lake Neosho was deposited during the highest stand of the sea during
the
Altamont depositional cycle, accounting for its uniformity and persistence.
It was less
subject to the kinds of disruptions that produced the great variability in immediately
underlying shallow-water strata. Slight fluctuations in sea level continued,
but the sea
had become so deep that these fluctuations did not significantly affect conditions
on the
sea floor.
The Lake Neosho at I-170 can be informally subdivided
into four subunits (photo):
1, calcareous shale; 2, blocky claystone; 3, phosphatic shale; and 4, bioclastic
shale
(ascending). The first three of these record progressively higher sea level,
reaching
a peak during deposition of subunit 3. Subunit 4 shows increasing agitation
of
bottom water as sea level fell.
1. Calcareous Shale. Calcareous shale lies at
the base of the Lake Neosho at I-170,
resting sharply on the underlying conglomerate and limestone unit. The shale
is
fissile and abundantly fossiliferous. Freshly scraped exposures reveal indistinct
festoon
cross bedding (photo), indicating deposition
under conditions of moderately strong
currents. Normally shales are not cross bedded. Shale at the base of the Lake
Neosho
may contain clasts of shale reworked from the underlying conglomeratic unit,
and would
therefore represent deposition under more energetic conditions than for most
shales.
Scattered phosphate nodules in the shale are irregular in location and orientation,
so
they were reworked from the underlying conglomerate and limestone unit. As
discussed previously, phosphate nodules in the conglomerate and limestone were
derived from yet older deposits, so the phosphate nodules in the calcareous
shale
subunit of the Lake Neosho have undergone at least two episodes of reworking.
The calcareous shale subunit is richly fossiliferous
(photo). Macrofossils are abundant
but are difficult to collect because much of the shell material was leached
away during
modern weathering, causing the shells to disintegrate readily when disturbed.
Microinvertebrates, however, can be extracted from the shale after treatment
with acetic
acid and sieving. Microfossils included, there are conodonts, agglutinated forams,
pelecypods, brachiopods (both inarticulates and articulates), gastropods, crinoids,
scaphopods, vertebrate material, platy calcareous algae, and carbonized plant
debris.
Abundant carbonaceous material indicates the shoreline was not far away.
The shale is mottled dark olive gray to brownish gray
and yellowish brown (photo),
with orange iron oxide stains covering weakly slickensided fracture surfaces.
The
fractures (photo) are compactional in origin,
having developed in response to
differential compaction beneath the Worland Limestone, which varies considerably
in
thickness near the middle of the outcrop on the west side of I-170 where the
shale is
exposed. These fractures are planes of weakness that may have promoted a
spectacular rockslide in 1999 when large blocks of limestone pushed well into
the traffic
lanes of I-170. Fissility, carbonate content, and fossil fauna disappear abruptly
at the top
of this subunit. A distinct upper contact can only be seen on freshly exposed
surfaces
swept clear of debris (photo).
2. Blocky Claystone. The blocky claystone is
noncalcareous and unfossiliferous, and
breaks with subconchoidal fracture. It lacks the broad fracture surfaces present
in the
underlying subunit. It contains rare clay-ironstone concretions (photo),
and in the
upper part contains widely dispersed, impure (clayey) phosphate nodules. The
claystone is dark gray where fresh, but mottled greenish gray, olive gray, maroon,
and grayish yellow where weathered. Yellowish orange iron oxide stains cover
many
exposed surfaces in all but the upper 10-15 cm of the subunit. In the area of
the
rockslide scarp, a prominent clay "dike" extends completely through
this interval and
into the underlying calcareous shale (photo).
This dike may be a result of relatively
recent shifting of strata associated with Quaternary topography, and such features
undoubtedly contribute to instability of the outcrop.
3. Phosphatic Shale. The upper part of the Lake
Neosho includes a conspicuous zone
of weakly fissile shale that is richly phosphatic. The phosphate occurs in nodules
and
thin laminae (photo). Nodules are subspherical
to oblate in shape, and as they become
more oblate they grade into laminae (photo).
The laminae are, in a sense, very thin, flat
nodules. The more spherical nodules are small--often about 1 cm in diameter.
More
irregularly shaped ones are often twice that size. The laminae are mostly less
than 0.5
cm in thickness, frequently pinching to nothing.
The shale is black where fresh, but upon weathering
becomes flaky and mottled
greenish gray and grayish purple with streaky yellowish orange iron oxide stains.
The
phosphate is light brownish gray, but often breaks along fractures that are
stained
yellowish brown to reddish brown by iron oxides. The lower contact of this subunit
is
indistinct, grading up from the underlying blocky claystone (photo).
The upper contact
is sharp and slightly undulatory due both to scouring by currents and to compaction
around phosphate nodules and small limestone lenses concentrated along the
contact (photo).
The phosphatic shale nearly lacks macrofossils, but
inarticulate brachiopods and fish
remains are occasionally found in phosphate nodules. However, microscopic conodonts
and agglutinated forams are abundant. The sea was apparently too deep for either
benthic algae and most invertebrates, but high productivity by planktic algae
in surface
waters supported a flourishing fauna of floating and swimming organisms. Organic
matter
in the form of dead swimmers and floaters rained down on the sea floor. Decay
of this
material quickly used up all available oxygen, whereupon unoxidized organic
matter
accumulated on the bottom to produce black, organic-rich shale. Phosphate from
the
breakdown of organic materials provided the source for precipitation of phosphate
nodules and laminae.
4. Bioclastic Shale. At the top of the Lake Neosho
is a layer of light greenish gray shale
that has a granular texture due to the abundance of marine skeletal material
including
large, well-preserved invertebrate fossils (photo).
The fauna is dominated by crinoids,
but also includes conodonts, agglutinated forams, brachiopods, gastropods,
pelecypods, branching bryozoans, nautiloid cephalopods, scaphopods, sponges,
echinoids, trilobites, and vertebrate material, along with small pieces of woody
material
and reworked carbonate-lined burrow structures (probably mainly the ichnogenus
Thalassinoides). This diverse fauna demonstrates there was an ample food
supply, and
wave agitation was strong enough to keep the bottom water well oxygenated. Initially
the sea was still too deep for prolific algal growth, but increasingly high
productivity and
favorable living conditions ultimately promoted higher rates of carbonate sedimentation.
The bioclastic shale contains abundant phosphate nodules.
Many of these have an
oblate shape, and lie at an oblique angle to bedding (photo).
These were probably
reworked from the underlying phosphatic shale that also contains oblate nodules.
Some
limestone lenses in the uppermost part of the bioclastic shale contain relatively
dense
concentrations of phosphate nodules (photo).
These may represent lag deposits
from strong currents that fragmented the more friable materials, sweeping finer
particles
away and leaving only the large, very durable nodules behind.
The contact separating the underlying phosphatic shale
from the bioclastic shale is very
sharp, and in places the top of the phosphatic shale is jagged and splintery
with deep
crevices filled by bioclastic material and reworked phosphate nodules (photo).
Local
scouring produced a slightly undulating contact, and currents concentrated skeletal
material into thin lenses of bioclastic limestone that filled low spots. The
change in
environmental conditions from quiet water during deposition of the phosphatic
shale
subunit to moderately strong currents during deposition of the bioclastic shale
subunit
may have been gradual. However, sediments that recorded such a gradation, if
ever
present, were removed when the current strength became great enough to scour
and
erode the sea floor. The upper contact with the Worland Limestone is gradational,
as
the matrix becomes increasingly calcareous and lenses of skeletal micritic limestone
make up an increasingly greater portion of the rock (photo).
In some places lateral
gradation from bioclastic shale to limestone is responsible for the base of
the Worland
descending in the section.
The thickness of the bioclastic shale varies from nothing
to nearly two feet. Part of the
decrease in thickness is due to lateral facies change to limestone, and part
is due to
bottom currents having eroded the bioclastic shale and underlying units. The
high
energy conditions indicated by scouring of the underlying phosphatic shale and
reworking of phosphate nodules into the bioclastic shale was apparently responsible
for local erosion of strata as low as the Mulberry Member, causing the Worland
Limestone to locally rest directly on any of the units between the Mulberry
and upper
Lake Neosho Shale (see discussion on the base of the Worland in the next section).
Lake Neosho Shale Member Summary. The Lake Neosho
Shale reflects two abrupt
changes in environmental conditions. The change from fossiliferous calcareous
shale to
unfossiliferous, non-calcareous blocky claystone (subunit 1 to subunit 2) was
abrupt,
showing a decrease in energy and oxygenation levels in the sea, with a corresponding
reduction in bottom fauna. This change was probably associated with rising sea
level.
The abruptness of the change suggests that when the sea's depth had increased
to a
critical value, circulation of bottom water suddenly decreased. Heckel (1991)
hypothesized that this was caused by development of density stratification in
the water
column. Higher in the section, the change from nearly unfossiliferous black
phosphatic
shale to highly fossiliferous bioclastic shale (subunit 3 to subunit 4) was
equally abrupt,
showing a return to favorable, well-oxygenated bottom conditions with corresponding
redevelopment of a diverse bottom fauna. This change was probably associated
with
falling sea level; the sea was no longer deep enough to allow stagnation in
a dense
zone of bottom water.
The phosphatic shale and bioclastic shale subunits
extend across most of the I-170
exposure. However, along some of the outcrop, only the bioclastic shale unit
is present
between the Worland and Mulberry, and elsewhere there is no Lake Neosho at all.
These are places where the Worland Limestone is especially thick. Most underlying
strata are missing there either because they were eroded prior to upper Lake
Neosho
deposition, or because they were not deposited in those locations (or possibly
because of a combination of these effects). Paleosol features, including root
mottling
and slickensides (although the latter may or may not be pedogenic), occur in
the
Mulberry where the conglomerate and limestone unit is absent (photo),
indicating at
least local subaerial exposure.
Worland Limestone Member
The I-170 exposure owes its character to the presence of the resistant Worland
Limestone. It is the most conspicuous rock unit at I-170, and has sheltered
the under-
lying, more easily weathered and eroded shaly strata so they can be readily
studied
(photo). The Worland is near the top of
the Pennsylvanian sequence in this part of
Missouri, although a few higher units are occasionally exposed during excavation
work along the I-70 corridor between I-170 and the area of County Road N.
The Worland Limestone was deposited as sea level fell
following the Lake Neosho
highstand. The progression from black phosphatic shale to bioclastic shale in
the upper
Lake Neosho shows the beginning of this sea level drop. In time it dropped so
much
that currents impinged on that sea floor, eroding lower parts of the Lake Neosho
and
even some of the underlying beds. Benthic algal and microbial communities appeared
when bright sunlight was able to reach the sea floor. These produced the large
volumes
of carbonate sediment that formed the Worland. Early during Worland deposition
low,
reef-like mounds of calcareous algae grew on the sea floor.
Throughout the member, thin shaly beds and partings
are interspersed with the
limestone (photo). These may have formed
by occasional influx of terrigenous
sediment, possibly during storms, or they may preserve episodes where no calcareous
sediment particles were deposited for a while, so the sediment consists primarily
of clay.
Clay was always being deposited in the Worland sea, but normally it is present
only as
an "impurity" in the limestone (limestones typically contain 10-15%
clay and other
non-carbonate minerals). Very thin shaly partings (mere surfaces) between limestone
beds may represent concentrations of insoluble materials left over from carbonate
dissolution at contacts between adjacent limestone layers.
The Worland reaches a maximum thickness of 12 feet along
the I-170 road cut. Locally
it decreases to about eight feet, with thinning occurring at the base of the
member; the
top of the Worland stays at the same stratigraphic level across the entire outcrop
(photo). Where the base of the Worland is
highest (and the Worland is therefore
thinnest), all or most of the underlying shaly units are present. Where the
Worland is
thickest, its base lies less than one foot above the varicolored claystone unit.
In those
places, all or most intervening strata except the lower part of the Mulberry
Member are
cut out by the Worland. Areas where the Worland is especially thin apparently
represent
topographic highs on the sea floor prior to Worland deposition. Areas where
the
Worland is especially thick show where it was deposited in topographic lows
produced
by erosion of underlying strata or by localized non-deposition of those strata.
At I-170 the Worland can be informally subdivided into
three subunits (photo): 1, hard
gray limestone; 2, yellow weathering (clayey) limestone; and 3, rooted limestone
(ascending order).
1. Hard Gray Limestone. The lower part of the
Worland includes the purest limestone in
the member. It is hard, blocky, medium gray, bioturbated limestone consisting
of whole
and fragmented skeletal material embedded in micritic matrix (photo).
Whole fossils are
abundant and taxonomically diverse, but are often difficult to collect because
the rock is
so hard. Fossils that were especially prone to fragmentation such as crinoids
and
calcareous algae are mostly broken into very small pieces. Brachiopods are the
most
conspicuous fossils, but crinoids, gastropods, fusulinid forams, bryozoans,
echinoids,
solitary rugosan corals, and sponges are also present, along with the encrusting
and
platy calcareous algae. The sediment hosted a thriving infauna of burrowers
that
thoroughly churned the sediment (photo).
Scattered algal mounds formed on the sea floor during
lower Worland deposition.
Portions of at least two are visible along the I-170 road cut, one south of
the I-70
overpasses on the west side of the highway (photo),
and the other on the north side
of the I-70 overpasses on the east side of I-170 (photo).
The mounds include both
laminar algal encrustations and wrinkly calcareous plates. In outcrop the algal
mound limestone has a distinct bluish gray hue and lacks well-defined bedding,
so it is easily distinguished from non-mound limestone (photo).
The algae, and
possibly microbial communities associated with them, were effective in trapping
fine
sediment that otherwise would have been swept away, thus giving rise to the
mounds.
In some respects the mounds may have resembled modern patch reefs (photo).
However, they are too low to have made wave-resistant buildups (they are only
2-3
feet thick), hence are not true reefs. Nevertheless, they were distinct high
points on
the sea floor, and adjacent beds drape over their tops and lap out against their
flanks
(photo). The mounds are bordered by zones
of debris washed off the mounds, including
fragmented algal debris, clasts of mound limestone, and other skeletal material
(photo).
Laminated encrusting algae that often grew around small tubular sponges are
common
just beyond the edges of the mounds (photo).
Rounded intraclasts of limestone, some
exceeding 2 cm in diameter, occur with the sponges.
Abundance of calcareous algae in the lower Worland indicates
the sea floor was well
within the photic zone. Modern encrusting red algae (Rhodophyta) in the family
Peyssonneliaceae live from just below low tide to 120 m, and are common in depths
as
great as 50 m (James and others, 1988). The fact that currents (probably mainly
from
storm waves) reaching the Worland sea floor were strong enough to rip up chunks
of
sediment and algal encrustations from the mounds suggests depths toward the
shallower end of this overall range, perhaps only a few tens of meters.
The hard gray limestone subunit varies from less than
one foot to about four and
one-half feet in thickness. The base of the member rises and falls along the
exposure,
accounting for this variability. Where the Worland is especially thick (the
hard gray
limestone is over four feet) some of the underlying units are absent, apparently
having
been eroded prior to Worland deposition. Where the Worland is especially thin
(hard
gray limestone is less than one foot), the section below the Worland is most
complete—
few if any if the underlying beds were removed prior to Worland deposition.
2. Yellow Weathering (Clayey) Limestone. Overlying
the hard gray limestone is softer,
clay-rich limestone that develops a conspicuous yellow tint upon weathering
and breaks
into splintery slabs. Skeletal material is less concentrated in this interval,
but fossils are
still common. For example, fusulinid forams are abundant in the lower part (photo),
and the upper part contains large brachiopods (photo),
fenestrate bryozoans, and
echinoid spines in piles that apparently reveal where the animals died and disintegrated
(photo). There was essentially no post-depositional
transport or other disturbance of
the spines, indicating very quiet water.
The upward change from relatively pure limestone to
clayey limestone in the Worland
probably reflects shallowing water and approach of the shoreline as the sea
regressed.
Clay derived by weathering and erosion of the neighboring terrain began to reach
this
location in the Midcontinent seaway. The water was nevertheless relatively still—much
more so than during deposition of the hard gray limestone subunit.
3. Rooted Limestone. The upper part of the Worland
consists of chunky, yellow
weathering limestone riddled by root molds (photo).
These are filled with clay-rich
carbonaceous material in the deeper parts, but are open higher up. Where fresh,
the
carbonaceous material is dark gray, but where weathered it has oxidized, and
the
clayey fill of the root molds is grayish purple (photo).
Modern weathering seems to be
responsible for the clays having slaked away in the open, upper parts of the
molds.
This part of the Worland documents a continuing drop in sea level. In fact,
the root
structures provide strong evidence for subaerial exposure as Worland deposition
came to an end. The environment may have resembled coastal marshes in
carbonate terrains today in Florida and The Bahamas (photo).
Reed-like salt-water
tolerant plants colonized the marshy terrain, producing root structures in the
upper
few feet of the member and helping to trap sediment due to a baffling effect.
Worland Limestone Member Summary. Worland Limestone
deposition began
when the sea became shallow enough for sunlight to reach the sea floor, with
consequent establishment of benthic algal and microbial communities. Algal mounds
grew where productivity was especially high and sediment trapping was efficient.
The water was well oxygenated and currents impinged on the bottom, making ideal
conditions for a flourishing marine bottom fauna, including burrowers.
The shoreline came closer to the St. Louis area as sea
level continued to drop. Clay-
rich limestone was deposited when fine-grained terrigenous sediment eroded from
near-by lowlands reached the St. Louis area. Current action was restricted in
the
shallow water. Ultimately, sea level fell so far that salt-water tolerant plants
colonized
the emerging terrain, producing rooted limestone at the top of the member. Continued
drop in sea level brought Worland deposition to an end.
Nowata Shale
As the shoreline regressed across the
region, terrigenous sediment deposited in bays
and estuaries and in river systems and coastal marshes formed the Nowata Shale.
The
Nowata is not well exposed at I-170, but excellent exposures were accessible
when the
intersection at I-70 and Highway N was constructed in 2002. At that location
the Nowata
consists largely of non-marine mudrocks including shale, claystone, mudstone,
and
siltstone, with a thin, impure coal (photo).
Sandstone at the base of the Pleasanton
Group exposed there is the highest Pennsylvanian unit in the St. Louis area
(photo).
Summary of Depositional History
Pennsylvanian strata exposed at the I-170 highway cut in St. Louis,
Missouri record
cycles of rising and falling sea level. These were caused by global climatic
fluctuations
that produced alternating glacial advances and retreats in higher latitudes.
Sea level
fell world-wide during cold times when glaciers advanced, and it rose when climates
warmed and the glaciers melted. Stratigraphic units at the I-170 exposure reflect
the changing depositional environments due to fluctuating sea level. Depositional
conditions for each unit are summarized in Table
1.
A mid-Bandera glacial advance in high latitudes caused
sea level to drop, exposing
recently-deposited sedimentary strata in the Midcontinent region to weathering,
and
producing a prominent paleosol. This formed the lowest unit exposed at I-170,
the
varicolored claystone unit of the Bandera Formation. As climates began to warm
and
sea level rose, marginal marine beds formed in coastal marshes (Mulberry Member,
Bandera Formation). When coastlines became more distant, limestone was deposited
in the clear water (algal limestone possibly equivalent to Farlington Limestone
Bed).
Climates eventually became colder again. Glaciers formed
in high latitudes, causing
sea level around the world to drop. As the sea withdrew from the St. Louis area,
previously deposited strata were eroded, producing shale and limestone clasts
that
were deposited in the conglomerate and limestone unit.
Shell-rich shaly beds formed (upper part of conglomerate
and limestone unit) as sea
level began to rise during the next warming. Dark-colored shales of the Lake
Neosho
Shale record the deepest waters of this marine cycle when organic matter and
clay
accumulated on the stagnant, nearly lifeless sea floor.
Circulation of bottom water improved during the next
drop in sea level. The bioclastic
shale subunit of the Lake Neosho was deposited when a diverse bottom fauna returned.
With continued shallowing, bright sunlight reached the sea floor, promoting
the growth
of benthic algae and microbial communities that produced large volumes of carbonate
sediment and the beginning of Worland Limestone deposition. At first, relatively
pure
carbonate was deposited in the well-circulated clear water (hard gray limestone
subunit,
Worland Limestone). Coastlines became closer as the sea continued to regress,
and
fine-grained sediment eroded from coastal lowlands reached the St. Louis area
(clayey,
yellow-weathering limestone subunit, Worland Limestone). Land emerged as the
sea
retreated further, and plants rooted along the marshy shoreline (rooted limestone
subunit, Worland Limestone). Terrigenous sediment brought in by streams flowing
across the newly emergent plains formed the Nowata Shale, bringing this depositional
cycle to an end.
REFERENCES
Cline, L. M., and Greene, F. C., 1950, A stratigraphic
study of the Upper Marmaton and
Lowermost Pleasanton Groups, Pennsylvanian, of Missouri: Mo. Geological Survey
and Water Resources, Rept. of Inv. 12, 69 p.
Heckel, P. H., 1980, Paleogeography of eustatic model
for deposition of Midcontinent
Upper Pennsylvanian cyclothems, in Fouch, T. D. and Magathan, E. R. Magathan
(eds.), Paleozoic Paleogeography of West-Central United States: Rocky Mountain
Section, Soc. Econ. Paleontologists and Mineralogists, 197-215.
Heckel, P. H., 1986, Sea-level curve for Pennsylvanian
eustatic marine transgressive-
regressive depositional cycles along Midcontinent outcrop belt, North America:
Geology, 14, 330-334.
Heckel, P. H., 1991, Thin widespread Pennsylvanian black
shales of Midcontinent North
America: a record of a cyclic succession of widespread pycnoclines in a fluctuating
epeiric sea, in Tyson, R. V. and Pearson, T. H., (eds.), Modern and Ancient
Continental
Shelf Anoxia: Geological Society Spec. Publ. No. 58, 259-273.
Heckel, P. H., 1994, Evaluation of evidence for glacio-eustatic
control over marine
Pennsylvanian cyclothems in North America and consideration of possible tectonic
effects, in Dennison, J. M., and Ettensohn, F. R. (eds.), Tectonic and Eustatic
Controls
on Sedimentary cycles: Soc. Econ. Paleontologists and Mineralogists, Concepts
in
Sedimentology and Paleontology #4, 65-87.
Heckel, P. H., editor, 1999, Middle and Upper Pennsylvanian
(Upper Carboniferous)
cyclothem succession in Midcontinent basin, U.S.A.: Guidebook for XIV-ICCP
(International Conference on the Carboniferous-Permian), Field Trip #8: Kansas
Geological Surv., Open File Rept. 99-27, 236 p.
Howe, W. B., and Koenig, J. W., 1961, eds., The stratigraphic
succession in Missouri:
Mo. Geological Survey and Water Resources, vol. XL, Second Series.
James, N. P., Wray, J. L., and Ginsburg, R. N., 1988,
Calcification of encrusting
aragonitic algae (Peyssonneliaceae): implications for the origin of Late Paleozoic
reefs
and cements: Jour. Sed. Petrology, 58(2), 291-303.
Kidder, D. L., 1985, Petrology and origin of phosphate
nodules from the Midcontinent
Pennsylvanian epicontinental sea: Jour. Sed. Petrology, 55(6), 809-816.
Lambert, L. L., Heckel, P. H., and Barrick, J. E., 2003,
Swadelina new genus
(Pennsylvanian Conodonta), a taxon with potential chronostratigraphic significance:
Micropaleontology, 49, in press.
Peppers, R. A., 1996, Palynological correlation of major
Pennsylvanian (Middle and
Upper Carboniferous) chronostratigraphic boundaries in the Illinois and other
coal
basins: Geological Soc. America, Mem. 188, 111 p.
Ritter, S. M., Barrick, J. E., and Skinner, M. R., 2002,
Conodont sequence
biostratigraphy of the Hermosa Group (Pennsylvanian) at Honaker Trail, Paradox
Basin,
Utah: Jour. Paleontology, 76, 495-517.
Swade, J. W., 1985, Conodont distribution, paleoecology,
and preliminary
biostratigraphy of the Upper Cherokee and Marmaton Groups (Upper Desmoinesian,
Middle Pennsylvanian) from two cores in south-central Iowa: Iowa Geological
Survey,
Technical Information Ser. No. 14, 71 p.
Thompson, T. L., 1995, The stratigraphic succession
in Missouri: Mo. Dept. Nat. Res.,
vol. 40 revised, 190 p.
MEASURED SECTION
Composite section of Altamont Formation and associated
strata exposed along both
sides of southbound lanes of I-170 near intersection with I-70.
NOWATA SHALE
13. Weathered shaly and silty beds poorly exposed in hill slope above Worland
Limestone; at near-by exposures this interval includes shale, claystone, siltstone,
sandstone, and coal (not measured).
ALTAMONT FORMATION
Worland Limestone Member
Rooted Limestone Subunit
12. Micritic limestone containing root molds that are open near top, and lower
down
are filled with clay and partially oxidized carbonaceous material that imparts
a purplish
hue to the weathered rock; upper part with open burrows reaches a thickness
of about
1.0 foot; lower purplish zone is also about 1.0 foot thick.
Yellow Weathering (Clayey) Limestone Subunit
11. Clayey micritic limestone, softer and lighter colored than underlying limestone;
breaks into splintery slabs where weathered and develops distinct yellowish
cast;
scattered fossils throughout; scattered shale interbeds up to 0.4 foot thick;
thickness
2.5 to 4.5 feet.
Hard Gray Limestone Subunit
10. Skeletal micritic limestone, dense, light to dark olive gray, bioturbated;
fossiliferous,
with local algal encrustations in lower part; scattered clay partings delineate
thin to thick
beds of limestone; algal mound and mound flank facies occur as lenses up to
3 feet
thick; algal plates embedded in micritic matrix impart distinctive bluish tint
to the algal
limestone; bedding of micritic limestone pinches out against rising flanks of
algal
mounds and drapes over their tops; mound flank facies consists of transported
encrusting and platy phylloid algae, lithoclasts, and invertebrate fossils,
all with stylolitic
contacts between clasts; overall thickness about 1.0 to 4.5 feet.
Lake Neosho Shale Member
Bioclastic Shale Subunit
9. Shale, calcareous, greenish gray, grainy; very abundant invertebrate fossils;
nodular
and lenticular micritic to bioclastic limestone lenses in upper part; lower
contact sharp
and apparently erosional; grades upward and laterally into Worland Limestone;
thickness
varies from zero to about 2 feet (truncated in places, and grades into overlying
unit).
Phosphatic Shale Subunit
8. Shale, black where fresh, mottled greenish gray and grayish purple with streaky
yellowish orange iron oxide stains where weathered; contains abundant phosphate
nodules and laminae; phosphate is brownish gray, weathering lighter, but often
breaking
along fractures that are stained yellowish brown to reddish brown; except for
vertebrate
material and inarticulate brachiopods in phosphate nodules, contains only microfossils;
gradational basal contact, sharp upper contact; thickness zero to 0.7 foot (truncated
in
places).
Blocky Claystone Subunit
7. Claystone, blocky, dark gray where fresh, mottled greenish gray, olive gray,
maroon,
and grayish yellow where weathered; yellowish orange iron oxide stains cover
many
exposed surfaces in all but the upper 10-15 cm; minor slickensides; apparently
unfossiliferous; sharp lower contact, gradational upper contact; thickness zero
to 1.6
feet (truncated in places).
Calcareous Shale Subunit
6. Calcareous shale, fissile (slabby), mottled dark olive gray to brownish gray
and
yellowish brown with orange iron oxide stains covering fracture surfaces; fossiliferous;
contains scattered phosphate nodules; slickensided; sharp contacts; thickness
zero to
1.6 feet (truncated in places).
BANDERA FORMATION
Conglomerate and Limestone Unit
5. Coquinoid limestone, with shells embedded in non-calcareous clay matrix;
light
greenish gray to maroon, depending on presence or absence of iron oxide cement;
slakes in water except where cemented by iron oxide; thickness zero to 0.6 feet
(may
be truncated at top, and laps out against highs on underlying unit).
4. Conglomerate consisting of claystone and limestone clasts up to boulder size
embedded in clayey matrix; limestone clasts are micritic limestone containing
encrusting and finely fragmented phylloid algae, invertebrate skeletal debris,
intraclasts,
and shale and claystone lithoclasts; claystone matrix is olive gray to dark
gray,
weathering maroon and grayish green; upper part contains nodular to even-bedded
micritic shell hash limestone, and thin undulatory lenses of algal fragment
micritic
limestone; where heavily weathered near top, limestone lenses and clasts are
altered
to structureless friable chert ("tripoli"); scour bedforms locally
at base and in upper part
of unit; thickness ranges from zero to about 4 feet (may be truncated in places
or
pinches out).
Mulberry Member
3. Claystone, dark olive gray to olive black where fresh, weathering to greenish
gray
and maroon; carbonaceous to calcareous, locally containing marine fossils; also
contains undulatory and anastomosing highly carbonaceous layers up to 0.1 foot
thick,
grading to impure coal ("smut"); phosphate nodules abundant at certain
levels, some
clearly transported; contains rounded claystone clasts, and scattered thin lenses
and/or
clasts of skeletal micritic limestone in lower part; thickness zero to 1.3 feet
(truncated
in places).
2. Slightly undulatory zone stained by reddish iron oxide; varies from only
a red or
brown-stained surface to a thin bed (up to about 0.1 foot) of clay stained by
iron oxide
grading to hard, relatively pure hematite; lower contact sharp, upper contact
tends to
be gradational; forms persistent marker, but locally absent where scoured.
Varicolored Claystone Unit
1. Varicolored blocky claystone, mottled and slickensided throughout, unfossiliferous;
olive gray with faint orange iron oxide stains on weathered surfaces in upper
0.5 foot;
various shades of gray (brownish gray, olive gray, greenish gray, and bluish
gray) with
yellowish orange rhizomorphs and mottles in middle; shades of "maroon"
(moderate
red, grayish red, and dusky red) with small yellowish orange rhizomorphs and
gray
root mottles in lower part; locally conspicuous very plastic light gray clay
layer near
middle of unit; maximum of about 3 feet exposed at base of highway cut.
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