Chapter 9: Early Ordovician Sea in Central Texas 1
Published:March 01, 1957
Valid paleoecologic inference demands consideration and proper interpretation of both paleobiologic and sedimentary data. With this in mind the writers have attempted to equate the observed lithic and biologic features of the Lower Ordovician rocks of central Texas with an existing environment. The rock sequence in question is entirely of limestone and dolomite (commonly cherty) and is known as the Ellenburger group. It is divided, from the bottom up, into the Tanyard, Gorman, and Honeycut formations, each formation including both limestones and dolomites.
A consideration of recent sediments potentially comparable to Ellenburger rocks leads to the conclusion that the limestones of the Ellenburger group probably in large part originated as chemically precipitated calcium carbonate muds and “muddy” pellet sands such as are forming today west of Andros Island on Great Bahama Bank.
Probably the dolomites of the Ellenburger group resulted from alteration at or below the sea floor and during diagenesis. Penecontemporaneous alteration presumably involves interaction between calcium carbonate sediments at the sea floor and magnesium compounds in solution or suspension in the contacting sea water. Later diagenetic alteration probably follows zones of permeability, perhaps with a preference for zones of persistent aragonitic composition in the muds. This may explain the irregularity of dolomitization and abruptness of transition to limestone shown by the more coarsely granular dolomites. The dolomites of the Ellenburger group are believed to indicate shoal water at the place of formation. Variations in the chemical equilibria within overlying waters are suggested by dolomitization, and especially by alternation of limestone and dolomite beds.
The cherts of the Ellenburger group appear to be roughly divisible into those that originated with the original sediments and those of complex origin. Euhedral dolomite crystals and dolomolds in the chert, and the texture and structure of the chalcedonic cherts, suggest penecontemporaneous origin. The spongy and highly quartzose dolomoldic cherts are probably of complex origin, with much of the quartz druse being secondary. The fact that in the Lower Ordovician rocks of the United States abundant chert is characteristically associated with dolomite, with chemically precipitated limestones, and with a dominantly molluscan faunal facies is probably a significant phenomenon the basis for which is not fully understood.
Disseminated quartz-sand grains are essentially absent from the lower third of the Ellenburger group (Tanyard formation), most abundant in the middle third (Gorman formation), and locally common in the upper third (Honeycut formation). The sand is believed to be wind blown. Its increasing abundance toward the southern and eastern parts of the Llano region seems to indicate that land lay to the south and east of the Ellenburger marine province.
Ellenburger biotas include a fairly large number and variety of gastropods and cephalopods, occasional trilobites and brachiopods, two genera of bivalved crustaceans, the chaetetid coral Lichenaria, isolated plates of cystoids and chitons, the dasycladacean alga Rhabdoporella, and probably colonies of blue-green algae through whose metabolic and sediment-binding activities stromatolites were formed. Soft-bodied mud-ingesting organisms were probably fairly abundant in Ellenburger strata, for faecal pellets are common even at stratigraphic levels where no other traces of life are found. Although there were probably times of widely endemic benthonic biotas, many of the Ellenburger fossils are believed to have been transported after death from local shelly communities in areas of firmer bottom than generally prevailed. Faunal changes at the formational boundaries are held to be best explained by eustatic or epeirogenic fluctuations, for essentially similar and contemporaneous faunal changes are widespread in the central and eastern United States.
The algally formed stromatolites are considered indicative of waters within the photosynthetic zone. As the penetration of light through these particular waters was probably inhibited by suspended flocculent lime mud, the average depth was probably quite shallow—perhaps less than 15 fathoms, with the bottom at places barely awash. Sedimentary structures such as mud cracks and ripple marks are in keeping with this suggestion. Local abundance of dasycladacean algae implies very shallow water for these places.
A consensus of sedimentary and paleobiologic evidence indicates that the Ellenburger province of the mid-Continent sea was (1) marine; (2) shallow, grading from shoal-water conditions at the south and east to somewhat deeper waters at the north and west; (3) warm; (4) with generally well-oxygenated waters but strongly reducing sediments below the sediment-water interface; (5) intermittently turbulent and turbid; (6) soft-bottomed at most times and places; (7) firmer-bottomed toward the west than at the east; (8) bordered by land at an unknown distance to the south and east; (9) visited by wind-blown sands in Gorman and Honeycut time; (10) varied as to depth, temperature, and chemical environment, both geographically and in time; (11) affected by eustatic or epeirogenic changes of level during the transitions from Tanyard to Gorman and from Gorman to Honeycut time; (12) locally colonized by abundant shelly benthos whose dead shells were at times widely distributed by moving water, but at times also supporting more widely distributed endemic biotas; (13) inhabited by numerous soft-bodied mud-ingesting organisms; and (14) closely comparable to the sea of the Great Bahama Bank west of Andros Island.
The report concludes with a comparison between the inferred attributes of the Ellenburger sea and those of the late Cambrian sea of central Texas and other Early Ordovician provinces of the mid-Continent sea.