THE UNIVERSITY OF KANSAS PALEONTOLOGICAL CONTRIBUTIONS July 10, 1974 Paper 70 STONE CITY AND COOK MOUNTAIN (MIDDLE EOCENE) SCAPHOPODS FROM SOUTHWEST TEXAS KENNETH A. HODGKINSON Exxon Company, U.S.A., Houston, Texas ABSTRACT At least 18 species of scaphopods are recognized in samples from the Stone City and Cook Mountain Formations (Claibornian Stage) of the Brazos River Valley in south- eastern Texas. These include 11 species of Cadtdus, 4 of Dentalium, and 3 of Fustiaria; 10 of the species are described and named for the first time. Apical fragments of scaphopods from these formations show that some scaphopods truncate their apexes by periodically discarding a significant portion of the test. This is especially evident in species of Dentalium. INTRODUCTION Middle Eocene exposures along the Brazos a few localities. Information on the nature of and Little Brazos Rivers in southeastern Texas sections exposed at these localities and on their yield numerous specimens of a wide variety of depositional environment, therefore, are recorded macrofossils, especially mollusks. Although this rather fully using personal observations and many fauna has long been known to paleontologists, previously published discussions. only a relatively few forms have received detailed Middle Eocene outcrops in the Brazos River analysis. Experience in the age and environmen- valley are located in the Gulf Coastal Plain of tal interpretation of well samples from Eocene southeastern Texas (Fig. 1) where good exposures subsurface sections in generally adjacent parts of are rare because the nonresistant strata are easily the Gulf Coast, however, demonstrates that some weathered and eroded. Seldom is more than sev- forms (particularly small types) occur in the sub- eral feet of section available. Thus, Stone City surface rather commonly and generally are not bluff and nearby exposures on Little Brazos River, associated with diagnostic microfossils. Accord- where exposures are nearly vertical and include ingly, increased understanding of such types and richly fossiliferous beds, contribute importantly to the precise definition of short-ranging taxa are understanding of Tertiary stratigraphy. needed to facilitate subsurface interpretations over Stone City bluff, the first-recorded Tertiary a wide area. fossil locality in Texas and the type locality of the Stone City Formation, extends some 1,500 LOCAL STRATIGRAPHY AND feet along the south or right bank of the Brazos ENVIRONMENTAL RELATIONSHIPS River (Fig. 2). This bluff (Texas Bureau of This and planned subsequent studies of selected Economic Geology locality 26-T-1) is near the elements in the middle Eocene fauna of south- bridges of Texas 21 and the Southern Pacific eastern Texas are based on detailed collecting at Railroad, 11.4 miles west of the courthouse in 2 The University of Kansas Paleontological Contributions—Paper 70 EXPLANATION Ok-, .....-‘,„..../.` ..... L- .1 L, -*. ..' t L-- N-n..- '" ,1 L 1-- Yegua Formation X4 S ARKANSAS ::11 Laredo S one City Formation Formation, o Cook Mountain i i:;; q.) Formation, and 1 Sparta Sand Lz El Pico SABINE Clay UPLIFT We ches Formation, Queen City Sand, and Reklaw Formation Big ford Includes some areas Formation of Sparta Sand Carrizo Sand AUSTIN San Antonio o RIO GRANDE EMBAY ME NT r IAREA EXAMINED FALCON DAM L _ 3-• • • 4 7-es 0 50 100 MILES -47C0 FIG. 1. Outcrop area of the Claiborne Group in Texas (after Eargle, 1968). Hodgkinson—Middle Eocene Scapho pods from Southwest Texas 3 FIG. 2. Stone City bluff (Texas Bur. Econ. Geology loc. 26-T-1), Little Brazos bluff (loc. 2I-T-1), and other Claiborne exposures in the Brazos River valley. Bryan, Brazos County and 11.5 miles northeast of A hard layer, named the Moseley Limestone the intersection of Texas 21 and 36 at Caldwell, by Renick & Stenzel (1936), forms the most Burleson County. It has become so important prominent bench at Stone City bluff. This layer that the beds and fauna at this locality are dis- is neither a true limestone nor a laterally con- cussed in most general descriptions of Coastal tinuous unit, but results rather from the weather- Plain Tertiary. ing and local cementation at the surface of a At low-river stage several feet of the Sparta fossiliferous glauconitic bed. A conglomerate Sandstone (Fig. 3), a nonmarine fluviatile or (possibly basal conglomerate) composed of glau- deltaic sand devoid of fossil mollusks, are ex- conitic calcareous claystone occurs locally just posed at the base of Stone City bluff. Discon- above the "Moseley Limestone." According to formably overlying the Sparta Sandstone is the Stenzel, this conglomerate marks a disconformity type Stone City Formation (61.1 feet). between the Stone City and overlying Cook The Stone City Formation is well exposed in Mountain Formations. Rocky Creek (also called Rock Branch) which Stenzel, Krause, & Twining (1957) suggested enters the south side of Brazos River approxi- that the Stone City beds at this locality were mately one-half mile upstream from the Texas 21 deposited in a lagoon or estuary where salinity bridge. The section here consists of alternating was almost normal. Glauconitic beds were de- dark carlxmaceous clays containing discontinuous posited in areas subject to waves or tidal currents silt and sand lenses and beds of fossiliferous cal- and, therefore, were well aerated. Intervening careous glauconitic sands. Concretions are abun- clays, on the other hand, are more representative dant at several levels. of a mud-flat environment. 4 The University of Kansas Paleontological Contributions—Paper 70 EUROPEAN the highway intersection at Caldwell, and 9.6 SERIES GROUP FORMATIONS MEMBERS STAGES miles west of Bryan. Stenzel (1936) and Scott //A' Ar Ar YEGUA Mount Tabor AUVERSIAN COOK Spiller MOUNTAIN Landrum (CROCKETT) Hurricane w z Wheelock LA 2=2=2222= oLA o z STONE CITY LA o SPARTA ap LA 7=22=22=2= -J <_I THERR1LL 0 C.) _o WECHES Viesca Tyus LUTET IAN =2=2=2= QUEEN CITY REKLAW Marquez Newby ZZ=ZEZZZ=ZZE CARRIZO —_____.,==== FIG. 3. Middle Eocene stratigraphic units of southeast Texas. Gaps produced by regional disconformities are in dicated by slanted lining. (After Stenzel, Krause, & Twining, 1957, p. 37.) Scaphopods, gastropods, pelecypods, serpulid worm tubes, ostracodes, foraminifers, bryozoans, hexacorals, spores, and pollen are common in the glauconitic fossil zones. Also generally present are teeth, bones, scales, otoliths, and dermal plates —0 of several kinds of fish and teeth and stinging barbs of rays. Remains of decapods, cephalopods, echinoids, ophiuroids, insects, chitons, barnacles, 100 charophytes, sponges, brachiopods, and plants are found more rarely. Trace fossils, such as cork- 200 screw-shaped burrows, are preserved commonly in the sediment. Approximately 24 feet of the Wheelock Mem- 300 ber of the Cook Mountain Formation (Crockett) disconformably overlies the Stone City Formation. Most of the Wheelock Member is covered by 400 alluvium at this locality but it is exposed in sev- eral small ravines. One of the largest exposures is in a small ravine near the southeastern corner —500 FEET ie,14° of the highway bridge. o Little Brazos bluff (Texas Bureau of Eco- nomic Geology locality 21-T-1) is on the east side of the Little Brazos River just upstream from the bridge of Texas 21 (Fig. 4). This is 1.8 miles FIG. 4. Location of sections on the Little Brazos River east of Stone City bluff, 13.3 miles northeast of measured and described by Scott (1963, fig. 4). Hodgkinson—Middle Eocene Scapho pods from Southwest Texas 5 (1963) described and illustrated the stratigraphy Road. It is now called Old Spanish Road by the of this bluff and that of older beds exposed on Texas Highway Department. the opposite side of the Little Brazos River. This One of the early travelers on this road was 30-foot composite section includes the best local the 28-year-old Dr. Ferdinand Roemer who, on exposures of the Cook Mountain Formation. July 29, 1846, "discovered" the fossiliferous beds At the base of this section is a one-foot iron- exposed at Stone City bluff. Several years later stone bed which contains concretions and crab (1848) he wrote, "While on a tour to the upper burrows. This is the "Little Brazos Limestone" Brazos I discovered, in the neighborhood of the of Stenzel (1936) and is about 30 feet stratigraph- town of Caldwell, strata of a ferruginous sand- ically above the top of the Stone City Formation stone with numerous well preserved Tertiary (Berg, 1970, p. 50). Downstream from Little shells. Crossing afterwards the Brazos, not far Brazos bluff are several exposures of glauconitic from this town, I had a still better opportunity clay and ironstone units containing large con- to see this formation along the steep hank of this cretions. river. It consists of alternating strata of brown, Most Cook Mountain exposures on the Little ferruginous sandstone and of dark-colored plastic Brazos River consist of medium gray fossiliferous clays, both teeming with fossils." shale and glauconitic marl with thin sand, ben- Since Roemer's time many geologists and tonite, and concretionary ironstone layers. On the paleontologists have visited the exposures on the basis of a foraminiferal study, Greenfield (1957), Brazos and Little Brazos Rivers and have re- suggested that the area was covered by brackish ported on the fauna and stratigraphy. In 1889, marsh to inner- and middle-neritic environments. R. A. F. Penrose, Jr. (1890) made a boat trip Scott (1963), from sedimentary structures and down the Brazos River examining and describing megafossils, interpreted this section as consisting various Tertiary bluff sections. His report con- of tidal mud-flat deposits grading upward into a tains the first systematic description of the section marsh environment. exposed at Stone City bluff. Heilprin (1891) Near the junction of the Little Brazos and identified and published on the fossils collected Brazos Rivers is a fine-grained sandstone contain- by Penrose.