Pepper formation

The Geology of Texas - Vol. 1
PEPPER FORMATION

  Nomenclature.--- The basal, non-calcareous, blue-purplish clay-shale which extends southwards from the Woodbine outcrop proper in McLennan County and underlies the Acanthoceras flags of the Eagle Ford is a distinct stratigraphic unit. It is separated from the underlying Grayson (Del Rio) by an unconformity, represented by a pebble conglomerate. Its top is marked by a sharp break in the character of sedimentation, the overlying Tarrant formation being an arenaceous flaggy limestone containing much fish debris, phosphatic bodies and fossil wood, and showing many evidences of shallow water deposition. The Pepper shale has a distinct fauna, now being studied. In the past its categorical reference to either the Woodbine or the Eagle Ford has only added to the confusion regarding its age, and for these reasons it is here separately named. It may be a part of the Woodbine, a question still open for debate. Taff (1575, p. 299), and later Stephenson (1534, p. 3), have expressed the idea that the "basal Eagle Ford shale" south of the Brazos is of Woodbine age. They brought forward no fossil evidence to support the idea. Taff thought the Woodbine must be present because he thought there was continuous deposition from Buda to Eagle Ford times, and Stephenson's correlation was purely lithological. The type locality of the Pepper shale is taken to be an exposure on a small branch of Pepper Creek, just south of the Belton-Temple highway, Bell County, Texas, and 1.6 miles east of the easternmost of two underpasses of the highway under the Santa Fe Railway.

  Outcrop.
--- The type locality of this shale was described in detail by Adkins and Arick (16, pp. 51-59). The basal reworked zone is about foot thick at most places. It is succeeded by a clay, having a rough conchoidal fracture and no fine shale lamination but with horizontal bedding planes at intervals of a few inches. This clay is carbonaceous and blackish blue when wet, but dull purplish gray when dry, especially in its lower part. It contains a fauna of ammonites, pelecypods, gastropods, and other fossils, preserved as delicate impressions, often crushed, or as thin films of shelly material.

  The member outcrops entirely across Bell County, and perhaps in McLennan County; however, much of the basal Gulf shale in McLennan County hitherto called "basal Eagle Ford shale" is different from the Pepper in fossils and lithology, and contains a large normal marine fauna, including the soft-shelled turtle, Protostega gigas Cope, other reptilian bones, various species of Inoceramus, one of them reaching 2 feet in diameter, and many other fossils (11, pp. 75-77). Nor is it certain that the intermittent, thin, purplish-black clays which underlie the base of the Woodbine sandstone in east Texas wells is the same as the Pepper, although the lithology is similar.  Drs. Winton and Scott have studied cores from the basal part of the Gulf series in wells of the Tarrant County Water Improvement District. Some of this material is similar lithologically and paleontologically to typical Pepper shale, but other cores contain fossils not yet found in the Pepper (1791, p. 36). The "basal Eagle Ford shale" near Austin is somewhat similar in lithology and fossils to the typical Pepper shale. A shale in this stratigraphic position is recorded as far south as Medina County.

  Paleontology
.--- At the type locality the Pepper contains a considerable fauna of gastropods, pelecypods, and a few ammonites preserved as delicate impressions. One ammonite suggests Neocardioceras, and another is a small Scaphites with nodes on the straight portion. Dr. Spath identified one ammonite as possibly a Euhystrichoceras. Other fossils from this locality are: Plicatula sp. aft. arenaria Meek, Tapes sp. aff. cyprimeriformis Stanton, Avicula sp. aff. gastrodes Meek, and Anchura sp. The pebble layer at the base of the formation contains reworked Exogyra arietina, Gryphaea mucronata, other oysters, fish remains, phosphatic and rounded quartz pebbles and other detritus. From the Tarrant County cores Dr. Spath identified an ammonite as possibly Metacalycoceras, and Dr. Scott identified a doubtful Mantelliceras (?) or Submantelliceras (?) . From a basal sandstone core near Van, Turritella aff. seriatim-granulosa Gabb (not F. Roemer) is known. In Bouldin Creek, South Austin, many gastropods and pelecypods occur.

  Dr. Spath states that "the largest specimen [from the cores of the Tarrant County Water Improvement District] probably is some "Calycoceras," but determinations are suggested only because the Cenomanian age is known." Of the fossils from the type locality of the Pepper clay, he says that it is just possible that one is a Neocardioceras. The best of all the specimens, a dwarf-form [desmoceratid] like Flickia or Adkinsia, but without suture line and also indeterminable does not help in dating this assemblage" (Dr. L. F. Spath, personal communication). Another type Pepper species is an unrolled, tuberculate Scaphites-like form with long straight portion, which Dr. Spath says is "not like anything known"; still another resembles keeled, ribbed Schloenleachids, and seems closest to Euhystrichoceras, but is still undetermined. The ammonites from this level are now being studied by the writer.

  Mrs. Helen Jeanne Plummer has kindly washed and examined material from the ammonite-bearing purplish clays above the reworked layer and about 2 feet above the base of the formation at the type locality, and has made the following statement:

  The thinly laminated, grey, and slightly variegated shale washes to a small concentrate in which small, white, and underdeveloped foraminiferal tests are very frequent. The most abundant form is a species of Ammobaculites in which the initial coil is very conspicuous. More rare forms consist of a minute Ammobaculites, very small Ammodiscus, and a very delicate Reophax.

 
Though arenaceous forms obviously developed in somewhat adverse environment do not constitute final evidence of geologic age, these forms are much more closely related to Upper Cretaceous faunas than to those of the Lower Cretaceous. These species have no relation to those of the Grayson or Del Rio formations, but are similar to species in the Eagle Ford and other Upper Cretaceous faunal groups. I feel no hesitancy in referring this shale to the Upper Cretaceous series.


  Age.--- The exact age of the type Pepper clay is still unknown. It overlies the upper Grayson (Del Rio) and underlies the Tarrant flagstone at its type locality. The basal contact is an apparently concordant one and is marked by a detrital bed of quartz pebbles and other coarse material, and the exact nature of the upper contact is unsettled (apparently it is concordant). The shales at the base of the Gulf series in Travis and Tarrant counties and in east Texas may be equivalent to the Pepper shale, but their age is still unknown.

  Mineral content.
--- Kelsey and Denton (899b) mention the following characteristics of the heavy mineral content of the Woodbine: titanite is rare; garnet is either absent or meager; zircon is not present in large amount.

  Paleontology and zonation
.—Knowledge of Woodbine zonation is so imperfect, the fossiliferous localities so scattered, and the fossiliferous levels in the formation so restricted, that only a general composite account of the paleontology is here outlined. Probably Woodbine time covers only a short interval of relatively rapid deposition, at least if judged by Spath's (1510) English zonation, where it would be practically limited to his vectense and diadema zones of the Upper Cenomanian, since the Buda (q. v.) is of Mantelliceratan age (Lower Cenomanian) and the basal Acanthoceras zone of the Eagle Ford is about the middle of the Upper Cenomanian (rotomagense zone). Subject to further confirmation of these correlations, the Woodbine would represent a lower portion of the Upper Cenomanian.

  Woodbine paleontology has been most studied near Lewisville, Denton County, and Tarrant, Tarrant County. Here the fossils are partitioned as follows:

5. Upper sandstone; hard to friable, ferruginous sandstone with shells and fish teeth; (zone of Acanthoceras n. sp., 2 miles north of Grandview) ; Exogyra columbella Meek, Ostrea soleniscus Meek, Ostrea carica Cragin, Exogyra sp., Barbatia micronema.... Feet

20-30
4. Upper shales, blue-black, gypsiferous, some seams of sand, lignite and ironstone; Ostrea carica Cragin.... 80
3. Lewisville beds (zone of Aguilera cumminsi) ; Aguilera cumminsi White, Ostrea carica Cragin, Ostrea soleniscus Meek, Exogyra ferox Cragin, Arca galliennei var. tramitensis Cragin, Trigonarca siouxensis (Hall and Meek), Barbatia micronema Meek, Pteria salinensis White?, Modiola filisculpta Cragin, Cytherea taffi Cragin, Cytherea leveretti Cragin; Turritella renauxiana d'Orbigny?, Cerithium tramitensis Cragin, Cerithium interlineatum Cragin, Natica humilis Cragin, Nerita sp. Cragin; Timber Creek near Lewisville, Denton County; localities in Hill County; about.... 100
2. Dexter sands; yellow ferruginous sandstone and cross-bedded sands, and brown siliceous ironstone seams and nodules containing dicotyledonous leaves; about.... 100
1. Basal clays; dark laminated clays; Mantelliceras (?) or Submantelliceras (?) pelecypods, gastropods; exposed on Sanger-Pilot Point road.... 5-25

  Between Denison and Sherman in the uppermost Woodbine, Hill records Cyprimeria, Ostrea soleniscus, Arca, and Exogyra columbella. In northwestern Fannin County in arenaceous marl of supposed Dakota age, Cragin records Ostrea lyoni Shumard, Exogyra ferox Cragin, and Cytherea taffi Cragin. Stephenson reports in a sand stated to be of either upper Woodbine or lower Eagle Ford age Metengonoceras dumbli, Metoicoceras swallovi Shumard and Acanthoceras sp. In a greensand at the top of the section (Woodbine?) at Pine Bluff, Lamar County, Hill records an ammonite (Scaphites?) and a crab attached to a log of lignite, and in the clays at the top of the bluff Axinea and Scaphites. From, Johnson and Hill counties Taff (1575, pp. 288–291) records abundant Lewisville oysters, pelecypods, and gastropods. The Timber Creek locality is described by Taff (1575, pp. 287–288) and by Hill (801, pp. 309–310).

  Woodbine invertebrates are listed by Hill (801, p. 314) and plants by Hill (801, p. 314) and by Berry (105).

  From the Woodbine in Texas there have been recorded: Cephalopoda 3 species, Pelecypoda 15, Gastropoda 5, and plants 43 (2 Gymnosperms and 41 Dicotyledons). It must be kept in mind that Berry considers the Arthurs Bluff plant-bearing beds (called Dexter sands of Hill) as more probably the "time equivalent of what is called Eagle Ford in the Austin section."