CHAPTER 1

Depositional environments and stratigraphy of late Eocene sediments, east-central Texas

By Thomas E. Yancey

Dept. Geology & Geophysics, Texas A&M University, College Station, TX 77843

ABSTRACT

Late Eocene strata in the Brazos River Valley area were deposited in parasequence-scale cycles containing sediments from a range of depositional environments. Cycles in the lower part of the section contain sediments deposited entirely in marine environments, while younger cycles contain sediments deposited in nonmarine and marine environments. In most lignite-bearing cycles, lignite occurs at the base of the cycle and is overlain by marine deposits. Most cycles are separated by an exposure surface, which is usually marked by plant rooting and/or development of a paleosol. Late Eocene formational units proposed for this area are too poorly known to be reliably identified and mapped.

INTRODUCTION

Late Eocene strata, which are part of the upper lignite-bearing interval (Yegua-Jackson) of the Paleogene section in Texas, are exposed in an arcuate belt extending from the Texas-Louisiana border to the Texas-Mexico border. The major lithology present in the outcrop belt is sandstone, deposited in laterally extensive sheets interbedded with mudstones and containing numerous thin to thick lignites and carbonaceous mudstones. Fisher and others (1970) characterized the eastern half of the outcrop belt as deltaic and the southern end as strandplain-barrier, although there is a relatively similar set of deposits with laterally extensive sandstones and lignites present along all but the easternmost parts of the outcrop belt. Near the Texas-Louisiana border, Late Eocene sediments are finer grained and are part of a set of deposits placed in the Yazoo shelf system. Along the rest of the outcrop belt, sand-dominated Late Eocene deposits extend downdip for 50-80 km (30-50 mi.) before changing to fine grained deposits of the open shelf depositional setting (Fisher and others, 1970).

Important contributions to the documentation of stratigraphy and depositional environments of Late Eocene deposits in east-central Texas, including the Brazos River Valley area, are presented in Sellards and others (1932), Ellisor (1933), Renick (1936), Russell (1955), Eargle (1959), Fisher and others (1970), Yancey and others (1993), Davidoff and Yancey (1993), and Yancey and Davidoff (1994). Other relevant studies include Eargle (1972) and Galloway and others (1979). Interpretations of Late Eocene environments of deposition presented in this report are based on recent studies of cores and deep cuts made by mining and dam construction.

LATE EOCENE STRATIGRAPHIC UNITS OF THE BRAZOS RIVER VALLEY

Three Late Eocene formations are recognized in the Brazos River Valley area (fig. 1) within a 360+ m (1200+ ft) section (fig. 2). Sellards and others (1932), Renick (1936), and Eargle (1972) provided the best systematic discussion of Late Eocene formations of Texas, but the lithologic character of formations in east-central Texas remains ill-defined because their type sections have been only briefly documented. Some formations differ substantially from the lithology of the type sections, located far from the Brazos River Valley. Fisher and others (1970) characterized the Late Eocene section in east-central Texas as being part of a major delta complex (Fayette

Delta System) in which the Caddell Formation (at the base) consists of prodelta and shelf fine grained deposits, the Wellborn and Manning Formations consist of delta plain sands, muds, and lignites, and the Whitsett Formation (at the top) consists of fluvial sands. This implies a genetic relationship between formations and predictable trends in lithology and depositional setting. In outcrop, the formations are not as distinct as suggested by the model and are strongly cyclic in character, with exposure surfaces separating the cycles.

CADDELL FORMATION

This formation consists of mudstones and sandstones, with mudstone as the dominant lithology (fig.2). The Caddell was deposited during a major marine flooding event and is distinct from other Late Eocene units in containing a high proportion of fine grained sediments. There is a coarsening-upward grain size trend in the formation, from a lower part composed of mudstone to laminated, sandy mudstones containing thin sandstone interbeds in the upper part. Observations in Brazos County show that the formation grades upward into well sorted, strongly burrowed shorezone sandstones forming the base of the Manning Formation. Renick (1936) noted that the Caddell Formation of the Brazos River Valley area is quite different from the fossiliferous, glauconite-rich shales and thin sandstones of the type area in east Texas. Mathews (1950) mapped a thin sandstone (which he named the Rock Prairie Sandstone) at the base of the Caddell Formation in Brazos County, but this sandstone body forms the top of the Yegua Formation in exposures along the Brazos River and is excluded from the Caddell formation.

MANNING FORMATION

This formation consists of a mixture of sandstones and mudstones, with numerous lignite beds and some volcanic ash deposits (fig. 2). It is distinct from other formations in containing thick (up to 2.5 m [8 ft]) lignite beds. Thick sandstones occur at several intervals within the formation, typically in close association with thicker, minable seams of lignite. Recognition and mapping of the Manning Formation on the basis of sandstones or lignite occurrence has been inconsistent. Variable lithification (including local silica cementation) makes the sandstones difficult to map and the lignites are poorly exposed, because lignite oxidizes readily near the ground surface in east-central Texas. The formation is nearly 300 m thick (1000 ft) in the Brazos River Valley, but has a similar grouping of lithologies throughout. The most detailed mapping of the Manning Formation in the Brazos River Valley is by Russell (1955, 1957) and the formation was studied from drilling records and core in the Gibbons Creek lignite mine by Eicher (1985).

Four sandstones of the Manning Formation in the Brazos River Valley area have been named separately (Mathews, 1950; Russell, 1955): the Bedias, Carlos, Tuttle (all in Grimes County), and the Yuma (in Brazos County). These are laterally extensive, shallow marine sandstones, but the names have been applied in an erratic manner to sandstones within different cyclic units and are not useful stratigraphic units at the present time, except in the immediate vicinity of their type sections. The lowest part of the Manning Formation in Brazos County contains a stratigraphic interval defined by Kennedy (1893) as the Wellborn Formation, with type section at Wellborn, just south of College Station. The Wellborn interval is too similar to overlying parts of the Manning Formation to be recognizable as a distinct lithologic unit away from its type area, which was briefly redescribed by Eargle (1959). Although Renick (1936) claimed that the Wellborn Formation should be recognized because of its utility in geologic mapping and Eargle (1959; 1972) used the name in south-central Texas, it has not been reliably identified outside Brazos County.

The base of the Manning Formation is placed at the base of the first thick sandstone (locally identified as the Bedias Sandstone) above the Caddell Formation. The top of the Manning Formation is placed above the highest thick sandstone (defined as Yuma Sandstone in Brazos County). Outcrop development of these boundary sandstone units is variable, so the lower and upper formation contacts are difficult to identify and locate.

WHITSETT FORMATION

The Whitsett Formation extends from the top of the Yuma Member to the base of the overlying Catahoula Formation. The formation consists of sandstone and mudstone, some volcanic tuff beds and some lignite beds near the base (in the Brazos River Valley). It differs from other formations in containing more tuffaceous sediments and nonmarine deposits. However, the boundary between the Manning and Whitsett formations is hard to determine because both formations contain a large proportion of sandstone. The Whitsett Formation type section is in south-central Texas, where the sandstones are marine in origin (Eargle, 1959), not fluvial.

LATE EOCENE SEQUENCE STRATIGRAPHY AND RADIOMETRIC DATES

The Late Eocene section contains numerous cycles of deposition, mostly of parasequence scale, grouped into three sequences. Sequence boundaries and flooding surfaces are identified on the basis of outcrop relations. The Caddell Formation is part of a transgressive systems tract whose sequence boundary lies within the Yegua Formation (Davidoff and Yancey, 1993; Yancey and Davidoff, 1994). A maximum flooding surface (the 38.8 downlap surface of Haq and others, 1988) is present close to the base of the Manning Formation (Yancey and others, 1993). A paleosol and exposure surface on top of the highest "Wellborn" sandstone marks the 38 sequence boundary of Haq and others (1988). The 37.0 sequence boundary of Haq and others (1988) occurs at the top of the lower nonmarine interval of the Somerville spillway section, with a maximum flooding surface (the 36.5 downlap surface of Haq and others, 1988) present in the lower part of the open marine sediments of that section (Yancey and Davidoff, 1994).

A radiometric date was obtained on a volcanic ash layer in the Gibbons Creek mine (at the base of the 3500 lignite seam), which may be a lateral equivalent of the ash at the top of the lower nonmarine section at Somerville spillway section (figs. 1, 5). Sanidine crystals dated by the argon-argon method yielded a date of 34.52 ± 0.19 Ma at 95% confidence level on error of the mean (person. comm., J. Obradovich, U.S. Geological Survey). This date indicates a late Priabonian age for the ash horizon, which corresponds to the upper part of planktic foram biochronozone P16, as determined by current radiometric calibration of Late Eocene biochronozones. This validates the placement of the 37.0 sequence boundary of Haq and others (1988) at the base of the 3500 lignite seam of the Gibbons Creek mine section, which probably correlates with the top of the lower nonmarine interval in the Somerville spillway section.

ENVIRONMENTS OF DEPOSITION

Detailed studies of depositional environments summarized in this report come from cores drilled in central Brazos County (lower part of Manning Formation), mine face exposures in the Gibbons Creek mine, Grimes County, and deep cuts at the Lake Somerville spillway section (upper part of the Manning Formation). These are representative of sedimentary deposits present in other Late Eocene cores and deep cuts in the area.

Transgressive-regressive cycles of deposition are the dominant feature of the Late Eocene section, with each cycle containing deposits from a range of depositional environments, mostly in marine environments. Cycles are separated by exposure surfaces and range from 3-15 m (10-50 ft) in thickness. Fully marine cycles are present in the lower part of the section, which contain marine deposits ranging from shoreline to mid shelf environments, while cycles in the upper part of the section contain non-marine deposits and marine sediments deposited in nearshore, shallow water environments. Lignite-bearing Late Eocene cycles resemble the model Manning Formation cycle described below.

A model Manning Formation cycle of deposition (fig. 3) has a thin nonmarine interval at the base (often containing wood and large, fairly complete leaf remains in claystone or sandstone), a lignite bed (or carbonaceous sediment with common woody plant debris), a thin fining-upwards sandstone or sandy mudstone, a cycle-center mudstone, and a thick coarsening-upwards sandstone (often intensely bioturbated) with a well-sorted zone of sand at the top that is capped by a rooted exposure surface that may have a thin paleosol zone.

Lignite in Manning Formation cycles is present at or near the base of the cycle. The lignite lies upon exposure surfaces (or on thin nonmarine units overlying the exposure surface) and has a conformable contact with overlying sediments. Overlying sediments are commonly burrowed, with burrows extending down into the top of underlying lignite. Thus, lignite deposition occurred near the marine shoreline and lignite was preserved where the shoreline migrated over the deposit. The occurrence of a palynofloral association with salt-tolerant species at the tops of Late Eocene lignites in the Brazos River Valley (Phillips and others, 1994) and the presence of high levels of pyrite in sediments associated with lignite in the Gibbons Creek mine, Grimes County (J. Horbaczewski, Navasota Mining Co., pers. comm.) supports this interpretation. This placement contrasts with the characterization of Yegua-Jackson lignites of east-central Texas as being deltaic or fluvial-deltaic in origin (Kaiser, 1974; Tewalt, 1986).

ENVIRONMENTS OF DEPOSITION OF THE GREENS PRAIRIE SECTION

Marine depositional environments are best known for a transgressive-regressive cycle in the base of the Manning Formation, which was sampled in a core drilled at the intersection of Greens Prairie road and Rt. 6, Brazos County (fig. 4). This is located 5 km (3 mi) northeast (along strike) of the Wellborn type section and contains the middle part and most of the upper part of the stratigraphic interval previously mapped as Wellborn in Brazos County (Renick, 1936; Mathews, 1950). The top of the core begins within a regressive sandstone unit, which is well exposed in adjacent outcrops, so complete coverage of the transgressive and regressive intervals is available for study. This cycle is a parasequence deposited during maximum marine flooding of the early Late Eocene (Yancey and others, 1993) and is typical of open marine depositional cycles present in the Caddell Formation and basal part of the Manning Formation. The sediments contain marine fossils throughout, although carbonate shell material is leached away. Yancey and Elsik (1994) described the palynology of sediments from the transgressive and maximum flooding portions of the Greens Prairie core. The lower parts of the cycle also contain notable concentrations of fish bone and scales and agglutinated foraminifera, indicating lower rates of sediment accumulation. The base of cycle consists of a thin layer of carbonaceous sand, containing common wood fragments and small molluscan fossils (as molds).

Depositional trends in the Greens Prairie parasequence correspond closely with changes in water depth, in both the transgressive and regressive portions of the cycle, and primary depositional structures are little disturbed by bioturbation. Trends are most clearly seen in the upper hemicycle, which have upward increase in sediment grain size, thickness of sand beds, and amount of bioturbation. Cycle-center mudstones contain numerous thin laminae of silt or very fine sand, slightly blurred by bioturbation but still visible. Overlying sediments contain thin interbeds of sandstone in mudstone (distal storm deposits), with sandstone layers increasing in number and becoming thicker. Higher in the section, sandstones with hummocky cross bedding are the dominant feature. These sandstones are overlain by a thick unit of well sorted, crossbedded sandstone, which is capped by a zone of extensively bioturbated sands.

Coarser grained sediments contain distinctive sedimentary structures that can be used to match deposits in the cycle with sediments of modern marine environments in the northwestern part of the Gulf of Mexico (Hill, 1985; Snedden and Nummedal, 1991). Cycle-center mudstones, containing small amounts (less than 1%) of sand or silt in discrete laminae, were deposited in a middle shelf setting in water depths 30 m (100 ft) or greater (Yancey and others, 1993), below storm wave base but within the zone of storm-generated currents on the seafloor. Intervals with thin sandstones in mudstone contain distal storm deposits, while intervals with hummocky cross-bedded sandstones contain proximal storm deposits, deposited on inner shelf settings below fair weather wave base and above storm wave base, between 8 and 30 m (26-100 ft) water depths. The sand is derived from shorezone environments and was moved onto the shelf by storm generated bottom currents, a transport mechanism indicated by the presence of sand-filled gutter casts in mudstone. The well sorted, cross-bedded sandstones in upper parts of the hemicycle were deposited in shorezone environments above fairweather wave base. The top of the sand body is strongly bioturbated, suggesting that the intertidal zone was inhabited by abundant burrowing organisms.

The top of this shoaling sandstone has an exposure surface marked by rooting traces, which at one site near the town of Wellborn, contains silicified root tissue of Eocene plants. Rooting commonly occurs in the tops of sandstones containing marine burrows, presenting some difficulty in determining the origin of biogenic structures in the sands. Exposure generally produced some cementation of the sand, so shoaling sandstone units in the Manning Formation tend to be the most strongly lithified units and are well exposed. Several of these sandstones contain silica-cemented beds.

ENVIRONMENTS OF DEPOSITION OF THE LAKE SOMERVILLE SPILLWAY SECTION

The upper part of Manning Formation is well exposed in the Lake Somerville spillway section (beside Yegua Creek, near the town of Somerville) (fig.1). This section (fig. 5) contains intervals of nonmarine and marine deposits. There are two marine cycles of deposition, with basal lignites, which are representative of lignite-bearing cycles throughout the Manning Formation. The succession of thick volcanic ash beds, lignite beds, and thick sandstones in the upper part of the section suggests a correlation of the Lake Somerville spillway section with the stratigraphic interval containing the 3500 and 4500 lignite seams of the Gibbons Creek mine (Eicher, 1985), Grimes County. Stratal cyclicity in the section is developed at two scales: a major marine flooding event produced the transgression at the base of the dominantly marine interval (the stratigraphic interval from 6-21 m [20-70 ft] on figure 5), and a smaller (parasequence) scale cyclicity that produced the two marine cycles contained within the dominantly marine interval. A sequence boundary is placed at the top of the higher rooted paleosol (6 m [20 ft] level on fig. 5) because this level marks the last horizon of sustained exposure above the fluvial and overbank deposits and underlies sediments deposited during rising baselevel conditions. This change from nonmarine to marine-dominated deposition is also recognizable in Brazos and Grimes counties. Placement of the maximum flooding surface at cycle-center level of the lower marine parasequence is based on the abundance of marine diatoms in that cycle and inferred low rate of sediment accumulation. The following descriptions are based on field and laboratory work conducted over several years.

The base of the section exposes a 2+ m (7 ft) thick bed of altered volcanic ash. The ash was moved by running water after airfall and redeposited as a fining-upwards set of stream and overbank sediments. While the fine ash is altered to clays, much of the sand-size glass shards and pumice clasts of this ash remain unaltered. All coarse material observed in the ash zone is of volcanic origin or plant debris or secondary concretionary material. This lower ash zone appears to be from a single major volcanic eruptive event. The ash zone is capped by a 1.5 m (5 ft) thick interval of clays, carbonaceous material, and small glass shards (nearly completely altered to clay).

The paleosol zone, a 2.5 m (8 ft) unit of stream and overbank deposits with two thick paleosols, overlies the lower ash unit. The lower paleosol is a fine grained mudstone that overlies siltstones containing lenses of small (cm-scale) cross-bedded sandstone, suggesting deposition by small streams. The upper paleosol formed on sand deposits, which have thin zones of carbonaceous wood debris and logs. Both paleosols contain common large and small (to 10 cm diameter) vertical root penetrations and stump casts (to 30 cm diameter), which also penetrate down into underlying sediments. The rooting horizons within the paleosols record the presence of a deeply rooted forest cover on the land before deposition of overlying sediments. The upper paleosol is directly overlain by a thick layer of altered volcanic ash.

The upper ash bed is 0.8 m (3 ft) thick and consists of altered volcanic ash, some unaltered glass shards and small pumice clasts, fragmented crystals, and clayey concretions formed around root traces. The larger glass shards are formed of stretched bubble-wall fragments. Average shard size decreases upwards, with glass shards to 0.5 mm diameter recovered from the base of the unit. Major element analyses (on electron microprobe in Dept. Geology, Texas A&M Univ., R. Guillemette, operator) of individual glass shards indicate a calc-alkaline composition for the glass. Averages on 24 replicate glass analyses from 10 cm above the base yield a SiO₂ content of 73.26% (0.48% SD), a Na₂O content of 1.93% (0.28% SD), and a K₂O content of 5.70% (0.36% SD). Averages on 24 replicate glass analyses from 30 cm above the base have a composition virtually identical to the 10 cm level, with SiO₂ content of 73.42% (0.49% SD), a Na₂O content of 1.89% (0.27% SD), and a K₂O content of 5.80% (0.30% SD). Finer glass shards have altered to clays, giving the unit a white color when weathered. The ash directly overlies a rooting and stump cast zone at the top of the underlying paleosol, indicating this is an airfall ash deposit that covered a forested ground surface. The top of the ash is stained brown from swamp waters and contains abundant siliceous monaxon sponge spicules (often bundled, occurring as fragments of sponge wall), indicating some reworking of ash by rising water levels. The top of the ash bed also contains numerous large stump casts, made visible by infilling of dark carbonaceous sediment into the whitish ash sediments.

The lower marine cycle is a thin parasequence cycle (4 m [13 ft]) which contains a basal lignite. The thin fining-upwards hemicycle culminates in cycle-center deposits of sandy mudstone, which in turn is overlain by a thicker coarsening-upward hemicycle of sediments progressing to well sorted, fine-medium grained sandstone at the top. The clastic sediments were deposited in open marine environments and are strongly bioturbated. The most offshore deposits at cycle-center contain abundant marine diatoms (Coscinodiscus; identified by Dr. J. Baldauf and Dr. I. Kaczmarska-Ehrman, Dept. Oceanography, Texas A&M University) and siliceous sponge spicules throughout. Sediments of the upper hemicycle have upward increase in grain size and sorting, increase in lamination and bedding, increase in primary sedimentary structures, and decrease in bioturbation, corresponding to upward increase in energy conditions associated with shoaling water conditions. The highest sands contain varied sedimentary structures, including bi-directional crossbeds, some climbing ripple sets, and very low angle crossbedding and plane bedding, probably deposited in a current-dominated shorezone setting. Shoreline wave energy was probably low, inasmuch as the zone of shorezone sands is thin and the sands contain common wood fragments. The top, well-sorted sandstones show rooting by plants and evidence of exposure.

This cycle of deposition began with deposition of volcanic ash covering a forested soil surface, which was itself forested and flooded by freshwater swamps, then the swamp was flooded by open ocean marine waters. Phillips and others (1994) report the occurrence of a palynofloral association with salt-tolerant species at the top of the lignite. The cycle was completed with deposition of the shoaling-upwards hemicycle, which was then exposed before deposition of the overlying cycle. Apart from a local scour horizon in the middle of the coarsening-upwards hemicycle, there are no distinct storm deposits, so the depositional setting is interpreted to be an area fronting a protected shoreline, not a stretch of coastline facing the full force of waves on the Gulf. This lower marine cycle records the time of maximum marine flooding for the upper part of the Manning Formation.

The upper marine cycle is a thicker parasequence cycle (9.6 m [32 ft]), deposited in a similar manner to the underlying cycle. The basal lignite is 1 m (3 ft) thick and contains two thin interlayers of volcanic ash. A lower 2 cm thick layer at 40 cm from the top of the lignite contains distinctive coarse kaolinite crystals (composition determined by X-ray diffraction and electron microprobe, Texas A&M University). An upper layer of variable thickness (0-10 cm) at 10 cm from the top (see Ruppert and others, 1994, for discussion of this layer) contains scattered peat-filled root penetrations and burrows extending downward from the overlying lignite and marine sandstones. The top of the lignite contains a palynofloral association with salt-tolerant species (Phillips and others, 1994). It is overlain by a fining-upwards hemicycle, culminating in cycle-center mudstone containing common marine diatoms (Coscinodiscus) and siliceous sponge spicules, deposited during maximum marine inundation. The thick (6.5 m [21 ft]) coarsening-upwards hemicycle consists of a progression from mudstone (with some thin sand laminae) to poorly bedded, strongly bioturbated argillaceous sandstone to well sorted, fine-medium grained, laminated, partly crossbedded sandstone. The well-sorted sandstone is locally silicified to quartzite, is abundantly rooted at the top, and shows evidence of exposure. (Silica cementation in Jackson sediments generally occurs in sands that have exposure surfaces at their top.) Sediments of this hemicycle were deposited under conditions of increasing energy associated with change from offshore waters below wave base to a wave-washed shorezone.

All marine deposits contain many pieces of wood and most parts of the cycles are strongly bioturbated, to the extent of destroying most of the original depositional fabric. Sandy zones within the lower portion of coarsening-upward hemicycle contain the highest diversity of burrows. The dominant burrow of the marine trace assemblage is a large (0.5-1 cm diameter) Planolites, which occurs with fewer examples of Thalassinoides, Paleophycus, Scolithus, Ophiomorpha, and Gyrolithes (identified with the help of Cheryl Metz, Texas A&M University). Some burrows with meniscate backfills are present in the more resistant clayey sandstone beds. A large majority of the burrows are horizontal to slightly inclined, although the dominance of this orientation is exaggerated because of flattening of burrow traces during compaction of the clayey sediments. The largest burrows and vertical burrows (to 30 cm height) occur in sandy beds. Scolithus burrows and Ophiomorpha burrows occur in the upper, well-sorted sandstones. Thalassinoides burrows are generally mid-sized (to 4 cm diameter), with minor branching. The simple Planolites burrows, walled Paleophycus burrows, and burrows with meniscate backfill are of closely similar size and orientation, and presumably produced by the same organisms.

The top lignite zone directly overlies the quartzitic sandstone bed and consists of top and bottom lignites separated by a middle interval of carbonaceous claystone. The middle claystone has a gradational basal contact with underlying lignite and contains many large carbonized logs (to 50 cm diameter) in random orientation, suggestive of deposition in standing water. The swamp environment present during accumulation of the bottom lignite layer produced modifications of the underlying sands, including reworking of a thin layer of sand to incorporate common pieces of wood debris and penetration of the top 20-30 cm of the sands with abundant large (to 2 cm diameter) rooting traces. These rooting traces have shallow to steeply inclined orientations, rarely in vertical orientation (in contrast to the vertical rooting prevalent in paleosols lower in the section). This suggests that plants growing on the sand surface were rooted in a saturated sediment with a high groundwater table.

The highest part of the section is a fluvial zone that includes a lower interval of interbedded thin mudstones and sandstones and a thick upper interval of fluvial channel sandstones. Sediments of the lower interval consist of irregular layers of fine-medium grained, laminated sand, with some small cross beds, and silty, porcellaneous siliceous mudstone, containing clasts of wood. The bedding and grain size characters suggest deposition in shallow or temporary settling ponds on the lower floodplain of large drainage systems. The sands at the top of the section are well sorted and crossbedded and contain large sets of tabular and trough crossbeds, often separated by thin drapes of silty clay. Crossbed sets often contain large, rounded, porcellaneous mudstone clasts at the base. The sedimentary structures of this interval are typical of fluvial channel fill deposits. Some channel fill sands contain horizons with concentrates of small (1-4 cm) pumice fragments, showing that volcaniclastics were a common component of the sediment load in the river system.

CONCLUSIONS

Late Eocene sediments exposed in the outcrop belt of east-central Texas were deposited predominantly in shallow marine environments, with intervals of non-marine sediments and beds of volcanic ash present in the higher parts of the section. Strata were deposited in parasequence-scale cyclic units, ranging from 3-15 m (10-50 ft) in thickness, that are separated by exposure surfaces. Most Late Eocene cycles resemble the model Manning Formation cycle illustrated in figure 3, which relates deposition to cyclic changes in sea level, involving flooding and progressive deepening followed by shoaling water conditions. Lignite layers are present at or near the base of cycles and are usually overlain by marine deposits. A strandplain-barrier model of deposition provides a better understanding of depositional setting than the fluvial-deltaic or deltaic models applied previously.

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Yancey, T.E., and Elsik, W.C., 1994, Paleoclimatological analysis of Upper Eocene core, Manning Formation, Brazos County, Texas: Gulf Coast Association of Geological Societies Transactions, v. 44, p. 776.

Introduction

Chapter 2.

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