Cretaceous Paleogeography: Implications of Endemic Ammonite Faunas
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Geological Circular 72-2 Cretaceous Paleogeography: Implications of Endemic Ammonite Faunas BY Keith Young BUREAU OF ECONOMIC GEOLOGY The University of Texas at Austin Austin, Texas 78712 W. L. Fisher, Director 1972 Geological Circular 72-2 Cretaceous Paleogeography: Implications of Endemic Ammonite Faunas BY Keith Young BUREAU OF ECONOMIC GEOLOGY The University of Texas at Austin Austin, Texas 78712 W. L. Fisher, Director 1972 Contents Abstract 1 Introduction 1 Paleogeographicsetting 1 Cosmopolitan-endemiccycles of the Comanchean 3 Trinity faunas 3 Fredericksburgcycle 6 Washita endemicfaunas— 8 Low generic diversity a key to endemism 8 Relationof endemismto depositionalcycles 9 Endemismand correlation 9 Conclusions 11 Acknowledgments 12 References 12 Illustrations Figures— Page 1. The Comanchean Shelf behind thebarrier reef 3 2. Block diagram illustrating the back-reeftopography for a part of Texas during theMiddle Albian 4 3. Paleogeographicfeatures of Texas during muchof the Comanchean ... 5 4. Diagrammaticrepresentationofrockscontaining endemicand cosmopolitanfaunas 6 Tables Tables— Page 1. CorrelationofComancheansectionsfor areasfromwhichformations arementionedintext 2 2. Alternationof endemic and cosmopolitanzones on theTexas. ComancheShelf 7 3. CorrelationwithEuropeanzones 11 CretaceousPaleogeography:Implicationsof Endemic AmmoniteFaunas Keith Young Abstract Endemic ammonite faunas evolved from cosmo- produced endemism. With the next basin adjust- politan faunas in a series of successive episodes ment the endemic faunabecame extinct,anda new overabout 35 million years of the Cretaceous of the cosmopolitan fauna migrated into the back-reef Gulf Coast of the United States. During basin— area, likewise evolving into an endemic faunainits basin-margin tectonic adjustments the Cretaceous turn. Six cosmopolitan-endemic cycles have been barrier reef was inundated or circumvented so that identified. Geological evidence suggests two or a cosmopolitan fauna entered the back-reef area. three additional cycles. Gradual isolation of the fauna behind the barrier Introduction Ammonites have long had a reputation for their thought to have been under the pressures of both cosmopolitan or world-wide representatives. In the adaptation to unique environment and isolation. last three decades nomenclatural refinement has The Comanche Series (Cretaceous) of Texas made the general cosmopolitan aspect of ammon- (table 1) contains alternating cosmopolitan and ites less noticeable to the casualobserver. Neverthe- endemic ammonite faunas. Endemism is partial, less, there are certain faunas that are more cosmo- or almost complete, depending on the degree of politan than others. Some faunas were cosmo- isolation. The writer's evidence deals almost politan because they were oceanopelagic. Other entirely with ammonites. The nomenclature and faunas were cosmopolitan because larval stages distribution of other mollusc groups indicate similar were oceanopelagic or because migration pathways phenomena, but the relationships of Texas Creta- were open for neritopelagic animals. Endemic ceousBivalviaand Gastropoda to extra-Texasforms faunas, in contrast to cosmopolitan faunas, are are not sufficiently documented to permit definite faunas restricted to a particular area. Although conclusions. there are many causes for endemism, the endemic Ammonites show a definite alternationof cosmo- faunas in the present discussion are thought to have politan and endemic faunas in the Comanchean; evolved (1) because of isolation by barriers to the Gulfian faunas were almost entirely cosmo- migration, (2) as adaptation to a unique environ- politan until early in the Maestrichtian, when an ment behind the barriers, or (3) some faunas are endemic Sphenodiscusfauna evolved. Paleogeographicsetting To develop fully, a strictly endemic fauna must up to 300 miles wide. Depositionon the shelf was be evolving in anisolated environment. Otherwise, dominated by carbonate sediments. The outer at least parts of the fauna will spreadto other parts margin of the shelf consisted of clay-free carbonate of the world,and cosmopolitanelements willenter deposits with rudist banks, algal masses, and other the supposedly isolated environment. The geogra- reef growths that represent a barrier reef complex phy of the Comanchean Gulf of Mexico (fig. 1) (fig. 1), extendingfrom southern Florida across the provided just such an isolating mechanism that present southeast Gulf of Mexico intoLouisianaand functioned episodically. across Texas roughly underlying the presentMio- TheComanchean rocks that underliemost of the cene outcrop (Sandidge, 1961, pp. 13-14; Winter, Gulf Coastal Plain of the United Statesand Mexico 1961; Tucker, 1962). At the RioGrande the shelf were deposited on a broad shelf;in Texasthis was margin extended south toward Monterrey in the Albian (Bishop, 1970; Bbse and Cavins, 1928, pp. 86-87); during Department of Geological Sciences, The University of Texas at the Aptian a westward trend is Austin. indicated by Smith (1970). Various, more spec- 2 Table 1. Correlation of Comanchean sections for areas from which formations are mentioned in the text SIERRA DE PICA- SOUTH TEXAS NORTH TEXAS BIG BEND, CENTRAL TEXAS CHOS, NUEVO SUBSURFACE OUTCROP TEXAS LEON, MEXICO Buda Buda Buda Grayson Del Rio Delßio Del Rio Main Street Pryor Cuesta Pawpaw del Georgetown Weno Santa Elena Crura Denton Salmon Peak Fort Worth Duck Creek ue 'ea lamic Sombrerotillo CO McKnight 5 -a Kainer Goodland Del Carmen LxJ West Nueces Walnut Walnut Tele hone uxy Canyon Tamaulipas Glen Corbula bed Glen Rose Glen Rose Rose Glen Rose Hensel Basal sands Absent Bexar o La Pena Cow Creek Cow Creek O Q_ en Hammett Hammett Sligo Cupido Sycamore Hosston Taraises tacular reef complexes branch off on various off inother directions between SanAntonio,Texas, tectonic features and confuse the configuration, and Monterrey, Mexico. Bose and Cavins (1928, tend to conceal the main shelf margin trend in pp. 86-87) had correctly interpreted this Albian northern Mexico,and have resulted in a variety of shelf trend some forty years ago,and stated that it reef and shelf margin trendsand interpretations for lay in the valley occupiedby the National Railroad both Texas and northern Mexico (Bishop, 1970, from Nuevo Laredo to Monterrey, between the fig.12;Smith,1970;Fisher and Rodda,1967,fig.1; Sierra Gomas, etc., on the west and the Sierra de Hendricks and Wilson,1967, fig. 4). None of these Lampazos, etc., on the east. In identifying the are entirely wrong, but most fail to identify the trend Bose and Cavins did not identify it as a shelf southward-trending Albian shelf margin because of margin but did recognize the change in depth of the more dominant reef complex trends branching water from shallower on the west to deeper on the 3 Fig.1. The ComancheShelf behindthebarrierreef east. That the shelfedgetrendsouth of Laredo may Wilson, 1967, p. 5). Although not always super- have had a different configuration during the imposed, at many times a barrier was sufficiently Aptian will not be argued, and it may be that the continuous across the entire Gulf Coast of Texas Albianshelf margin of Bose and Cavinsrepresentsa and northern Mexico to prevent the entrance of shallowing Tamaulipas ridge with deeper water cosmopolitan ammonite species into the back-reef deposits againfarther west. area. Perhaps it should beemphasized that a single Although that part of the barrier overlying the invasion may not be sufficient to provide a viable Middle Trinity, in Texas, was first designated the breedingpopulation. Continuous interrelationships Stuart City trend by Winter (1961, p. 17) (figs. 2 andcommunication withcontemporary descendents and 3), later interpretations show many barriers of ancestral populations may sometimes be neces- (fig. 4) not always superimposed (Hendricks and sary to produce viable populations. Cosmopolitan-endemic cycles of the Comanchean Trinity faunas the Hosston (Lower Trinity) are cosmopolitan, no ammonites are known from the Sligo Formation The Trinity rocks were deposited ina variety of or its back-reef equivalents. These formations shallow marine to near-shore terrestrial environ- occur only in the subsurface, all of the ammonites ments. Many of these environments, even the available are those brought up from cores for oil shallow marine, were not hospitable to animals tests, and faunal documentation is incomplete. adapted to marine waters of normalsalinity. Conse- If the sediments behind the Sligo barrier reefs quently, the record of marine faunas is often (Hendricks and Wilson, 1967, p. 5) are like sedi- incomplete. Although allknown ammonites from ments in similar paleogeographic positions during 4 Fig. 2. Block diagram illustrating the back-reef topography for apartof Texas during theMiddleAlbian the early Cretaceous, endemic faunas probably politan forms disappear, and the ammonite fauna evolved behind these barriers but have not been of the Cow Creek Limestone consists almost collected (fig. 4and table 2). entirely of the endemic speciesDufrenoyia justinae = With the beginningof the Middle Trinity (Lozo (Hill, 1893) [= D. texana (Burckhardt) D. and Stricklin, 1956), the Pearsall Formation of roemeri (Cragin)]. Collection failure does not many subsurface workers, there appears for the seem to be involved since only a half dozen first time at the outcrop in the Hammett Shale ammonite specimensareknownfrom theCow Creek an Aptian cosmopolitan ammonite fauna contain- Limestone, except for the many, many specimens ing Cheloniceras,Procheloniceras,Eodouvilleiceras,