- © 2000 by AASP Foundation
Cooling paleoclimates over the period ca. 42 Ma to ca. 33 Ma were the driving force for the gradual disappearance of tropical and subtropical elements and the appearance of cooler elements in the palynofloras of east Texas. As a consequence, 11 palynomorph assemblage zones, one range zone and three biohorizons are recognized in 209 samples from 15 sections of the Crockett, Yegua, Caddell, Manning and Catahoula formations. From oldest to youngest, the succession of biozones is: middle Eocene Nuxpollenites crockettensis Assemblage Zone, Bombapollis texensis Assemblage Zone, Bursera Assemblage Zone, Friedrichipollis claibornensis Assemblage Zone, and Reticuloidosporites pseudomurus Assemblage Zone; late Eocene Sequoiapollenites Assemblage Zone, Rhizophora FAD, Rhizophora Assemblage Zone (which contains an ancestral Mutisieae pollen type in its lower part here designated the Mutisiapollis FAD and Mutisiapollis Zone), Nudopollis terminalis Assemblage Zone, Nudopollis terminalis LAD, Bombacacidites Assemblage Zone, and Pseudolaesopollis ventosus Assemblage Zone; and the early Oligocene Hypoxylonites Assemblage Zone. The age of the lower Catahoula Formation is early Oligocene based on the occurrence of Kallosphaeridium biornatum from a locality northwest of Huntsville, Texas. A short-term cooling event near the end of the Eocene set the stage for further changes in the early Oligocene. Use of these assemblage zones greatly increases the age resolution available for late middle Eocene to early Oligocene strata of the Gulf Coast.
The middle Eocene Claiborne Group and late Eocene Jackson Group of the northwestern Gulf Coast region are best documented from exposures in the Brazos River Valley and adjoining areas of central and east Texas. Exposures of these strata and strata lower in the section have been described in many field trip guidebooks (e.g., Smith, 1956, 1958; Russell, 1960; Berg, 1970; Zingula, 1974, 1984; Kersey and Stanton, 1979; Yancey, 1984) and summarized in regional reports (Fisher et al., 1970; Yancey and Davidoff, 1994). Numerous samples have been taken from this section for the study of palynomorphs: a number of those studies have been published (e.g., Elsik, 1974; Frederiksen, 1981; Ethridge Glass et al., 1987; Jones and Gennett, 1991; Gennett, 1994; Raymond et al., 1997) and some sections have been the subject of palynological theses (e.g., Ethridge, 1976; McMahon, 1997; Sancay, in progress).
Compilation of the occurrence data from these and unpublished studies of the senior author is part of the ongoing structure of the Gulf Coast PalynoDataBank (GCPDB) which covers most of the Cenozoic in the northwest Gulf of Mexico Basin. Sufficient data are now available to provide greater resolution in the interpretation of the paleoclimate for the interval from the late middle Eocene to the early Oligocene (Yancey et al., in press). For that study, 209 samples from 15 sections (Text-Figure 1⇓; Appendix) track a changing paleoclimate that cooled from tropical during the late middle Eocene to marginally subtropical in the early Oligocene, and included a cooling event near the end of the Eocene.
A plot of the occurrence data from those samples reveals that during the interval of ca. 42 Ma to ca. 33 Ma a major portion of the tropical and subtropical indicators disappear from the Gulf Coast and a number of cooler elements appear. The changing character of those palynomorph assemblages over time allows the definition of 11 assemblage zones, one range zone, and three biohorizons (Text-Figure 2⇓).Twelve palynomorph zones and three biohorizons over a time span of approximately 9 million years (Text-Figure 3⇓) offers a new potential for correlation of Gulf Coast upper Paleogene strata. These zones and biohorizons add to the late Eocene–early Oligocene Gulf Coast palynostratigraphy developed by Tschudy (1973, 1975), Elsik (1974) and Frederiksen (1980).
Much of the Claiborne and the lowermost Jackson strata of east Texas are known to be marine or marginally marine in origin (e.g., Atlee et al., 1967), but most of the Jackson Group strata were assumed to be deltaic or nonmarine because of the presence of many lignite beds and the lack of carbonate in the strata. However, the presence of characteristic marine trace fossil assemblages, common occurrence of tidalite sedimentary structures, and some siliceous marine microfossils (Yancey, 1997) indicate most of the Jackson Group strata are marine in origin. These strata lack the calcareous microfossils historically used in age determination of strata in the Gulf Coast because of dissolution of the calcareous material. The loss of these calcareous fossils/microfossils in the late middle Eocene to early Oligocene of the Brazos River Valley and adjoining areas is mitigated by the occurrence of marine palynomorphs (dinoflagellates and foraminifera linings) in the section (Text-Figure 4⇓). For example, age of the lower Catahoula Formation at the Riverside quarry in Walker County can be determined to be early Oligocene based on the occurrence of the marine dinoflagellate Kallosphaeridium biornatum. The potential exists for further refinement of the present biozones with fuller documentation of the common dinoflagellates (e.g., in the Yegua Formation and Caddell Formation at Koppe Bridge) and the portion of the profuse and diverse angiosperm microflora that is currently relegated to ‘angiosperm pollen undifferentiated.’
A number of the palynomorphs present in parts of the Brazos River Valley section are reworked from older strata, although not in the diversity found by McLean (1968) in the older Paleogene of the eastern Gulf of Mexico. Reworked late Paleozoic Densosporites and Lycospora occur in strata exposed in the Turkey Creek section, indicating that Carboniferous strata were exposed in central Texas during the late middle Eocene. Reworked Mesozoic, particularly Cretaceous taxa, are present in middle and late Eocene and early Oligocene strata. These include Aquilapollenites, Araucariacites, Classopollis, Dinogymnium, Ephedra voluta (in part), Odontochitina and Wodehouseia. The general presence of these taxa indicates widespread exposure of Cretaceous strata across the sediment source area in central and northern Texas. While the presence of Aquilapollenites was already known for central Texas (Evitt, 1973), reworked Aquilapollenites and Wodehouseia together suggest a fuller component of the Aquilapollenites province in the sediment source area. The reworked palynomorphs from Paleocene and early Eocene strata include spinose forms of Selaginella, Spinaepollis spinosus, Thomsonipollis magnificus and Tricolporopollenites baculoferus. This suggests that early Paleogene Midway Group and Wilcox Group strata were available sources for reworked material, especially during sealevel lowstands. Most of the reworked early Paleogene palynomorphs occur in middle Eocene strata.
The biostratigraphy of the upper Claiborne Group, the Jackson Group, and the lower part of the Catahoula Formation is greatly enhanced by use of the following new biozones and biohorizons. All but one of the zones are assemblage zones based on the association of a few palynomorphs, and named for the most characteristic taxon of each interval. Many of the assemblage zones are in proximity to potential biohorizons consisting of last appearance datums (LADs) and first appearance datums (FADs), but are not defined here with the exception of two FADs and one LAD. The Rhizophora FAD is named because it coincides with the base of the Rhizophora Assemblage Zone and has the potential for long-distance correlation. The Mutisiapollis FAD is defined because it marks the first appearance of Asteraceae pollen in the Gulf Coast and coincides with the Rhizophora FAD. The Nudopollis terminalis LAD is defined because it appears to be of regional significance. All of the named assemblage zone taxa are potential range zone markers.
LATE MIDDLE EOCENE (BARTONIAN)
Nuxpollenites crockettensis Assemblage Zone
The Nuxpollenites crockettensis Assemblage Zone is characterized by the presence of the angiosperm pollen Enopadios reticulatus, Nothofagus tschudyi, Nuxpollenites claibornensis and Nuxpollenites crockettensis. Enopadios reticulatus and Nuxpollenites spp. are good markers of a restricted stratigraphic interval within the upper middle Claiborne Group (Elsik, 1973, 1974) and the potential of last appearance datums (LADs) exist for those and Nothofagus tschudyi. This assemblage is present in the Stone City and Wheelock members of the Crockett Formation. In terms of the paleoclimate, this assemblage lived during the middle Eocene high paleotemperature interval of Andreasson and Schmitz (2000), with much of the palynoflora reflecting paleotropical vegetation, e.g., Dicolpopollis, Bursera, Friedrichipollis claibornensis, Symplocos aff. S. gemmata, Ficus, Anacolosidites, and Nypa (Text-Figure 3⇑).
Bombapollis texensis Assemblage Zone
The Bombapollis texensis Assemblage Zone is based on the presence of Bombapollis texensis, Dicolpopollis sp., Pandaniidites spp., and Triatriopollenites aroboratus in the absence of Enopadios reticulatus, Nothofagus tschudyi and Nuxpollenites spp. This assemblage occurs in the uppermost Crockett Formation and possibly in the lower Yegua Formation below the level of the strata exposed at the Turkey Creek locality. The paleoclimate continued to be the driving force in the disappearance of tropical elements from the microflora, consequently B. texensis, Dicolpopollis sp., Pandaniidites spp., and T. aroboratus occur to at least the top of this interval but not in the Yegua Formation at Turkey Creek; all have the potential for designation as biohorizons at the top of the Bombapollis texensis Assemblage Zone or higher in the section.
Bursera Assemblage Zone
The Bursera Assemblage Zone is based on the association of Bursera spp., Friedrichipollis claibornensis and Pistillipollenites mcgregorii below occurrences of Juglans spp. and an undescribed Osmundacidites spp. complex, and above the last occurrences of Bombapollis texensis, Dicolpopollis sp., and Pandaniidites spp. Cooling paleoclimate is indicated by the first appearances of new taxa above the top of this assemblage zone.
Friedrichipollis claibornensis Assemblage Zone
The Friedrichipollis claibornensis Assemblage Zone is characterized by the combined occurrences of Bursera spp., Friedrichipollis claibornensis and Pistillipollenites mcgregorii with the lowermost occurrences of Juglans and the undescribed Osmundacidites spp. complex. The Friedrichipollis claibornensis Assemblage Zone is present within the Yegua Formation strata exposed in the Turkey Creek section. Bursera, Friedrichipollis claibornensis, and Pistillipollenites mcgregorii have not been seen higher than the middle Yegua, signifying the loss of three more tropical elements from the palynofloras and the potential for three more LADs.
Reticuloidosporites pseudomurus Assemblage Zone
The Reticuloidosporites pseudomurus Assemblage Zone is characterized by Hoheria sp. (a very small Malvacipollis type), Reticuloidosporites pseudomurus, and Symplocos aff. S. gemmata. The assemblage zone is above the last occurrences (e.g., potential LADs) of Bursera spp., Friedrichipollis claibornensis, and Pistillipollenites mcgregorii. The Reticuloidosporites pseudomurus Assemblage Zone occurs within the upper Yegua Formation in the Koppe Bridge section, but not in the Caddell Formation, suggesting potential LADs for all three of its characteristic species.
LATE EOCENE (PRIABONIAN)
Sequoiapollenites Assemblage Zone
The Sequoiapollenites Assemblage Zone is characterized by occurrences of Sequoiapollenites, Ficus spp., Glaphyrocysta intricata, and Nudopollis terminalis below the biohorizon Rhizophora FAD, and before the appearance of cooler, drier elements of the palynofloras. The assemblage zone is above the last occurrences of Hoheria sp., Reticuloidosporites pseudomurus, and Symplocos aff. S. gemmata, and below the Mutisiapollis FAD and the Rhizophora FAD. The Sequoiapollenites Assemblage Zone is seen in a short interval of the basal Caddell Formation exposed within the Koppe Bridge section.
The Rhizophora FAD is a biohorizon that is defined as the first appearance datum of Rhizophora in the Gulf Coast. The pollen morphology is distinctive, and matches very closely the pollen of extant plants of the genus. Rhizophora appears in the lower Caddell Formation in the Koppe Bridge section, very near the base of the late Eocene. Muller (1981) attributed the oldest Rhizophora in the world to the late Eocene of the Caribbean area, citing reports of the form species of Rhizophora pollen, Zonocostites ramonae, by Germeraad et al. (1968). It is expected that this datum will be of regional magnitude.
Rhizophora Assemblage Zone
The Rhizophora Assemblage Zone is characterized by the sporadic occurrence of the pollen of the subtropical black mangrove, Rhizophora. Associated palynomorph taxa in this assemblage zone include Ficus, Sequoiapollenites, and Glaphyrocysta intricata. The Rhizophora Assemblage Zone encompasses most of the Caddell Formation. The disappearance of Ficus from the section is attributed to the cooling paleoclimate. The appearance of Rhizophora was probably due to migration into the area, although an ancestral type, Pseudolaesopollis ventosus, was already, and continued to be, present. Modern Rhizophora are tropical to subtropical (Watson and Dallwitz, 1992); tolerance of somewhat cooler paleotemperatures would have afforded an advantage over purely tropical mangrove elements.
The first appearance datum of Mutisiapollis spp. is in the lower Caddell Formation at the level of the Rhizophora FAD, e.g., it is a good biohorizon for the early late Eocene. Mutisiapollis is not seen in the late middle Eocene Yegua Formation.
The Mutisiapollis Zone is a range zone based on occurrences of Mutisiapollis spp.; the zone occurs from the base of the Rhizophora Assemblage Zone to about the middle of the Caddell Formation exposed at Koppe Bridge and sampled in the Greens Prairie core. The base of the Mutisiapollis Zone is set on the Mutisiapollis FAD and the Rhizophora FAD.
The undescribed ancestral Mutisieae pollen types that occur sporadically through the lower to middle part of the Rhizophora Assemblage Zone are referred to Mutisiapollis spp. Both first and last occurrences of the pollen are in the Caddell Formation. The appearance of ancestral Mutisieae pollen in the Caddell follows the appearance of grass pollen (Gramineae) in the uppermost Yegua Formation sample at Koppe Bridge; both are indicative of cooling, drying paleoclimate. The occurrence of this ancestral Mutisieae type is the oldest record of the Asteraceae in the Gulf Coast.
Nudopollis terminalis Assemblage Zone
The Nudopollis terminalis Assemblage Zone is named for the presence of any of the Nudopollis terminalis species complex in combination with Anacolosidites spp. and Gothanipollis spp. Ficus, Sequoiapollenites, and Glaphyrocysta intricata are absent. Nudopollis terminalis does not occur above this assemblage zone, and the potential is also good for LADs amongst the other taxa in the assemblage. The Nudopollis terminalis Assemblage Zone is seen in Manning Formation strata at the Gibbons Creek mine.
Nudopollis terminalis LAD
The LAD of Nudopollis terminalis appears to be a good biohorizon of regional importance in the upper middle Manning Formation, although it occurs to near the top of the Yazoo Clay in Mississippi and Alabama (Text-Figure 5⇓). Gennett (1996) found N. terminalis in the Manning strata exposed in the Gibbons Creek mine, Grimes County. However, N. terminalis has not been seen in the Manning at the Lake Somerville section (Elsik, study in progress; Sancay, study in progress), indicating that there is expanded section in the Lake Somerville area. The occurrence range of N. terminalis also has a bearing on the age of the San Miguel lignites in south Texas, where N. terminalis is absent (Gennett, 1993). The FAD of N. terminalis is much lower in the section; good specimens occur in the lignites of the upper Paleocene Calvert Bluff Formation (Elsik, 1968). Nudopollis terminalis is one of the last species of the Normapolles Complex to occur in the Gulf Coast region and persists into the late Eocene (Tschudy, 1975).
Bombacacidites Assemblage Zone
The Bombacacidites Assemblage Zone is named for the combined occurrences of Bombacacidites spp., Gothanipollis spp., Plagianthus? spp., and Nypa spp. above the Nudopollis terminalis LAD. Among these are some of the last tropical floral elements present in the section, which then disappear from the Brazos River Valley section, and all have good potential for biohorizons in the uppermost Jackson Group. They are last seen in strata of the Manning Formation in the Lake Somerville section, where there is good evidence for a short-term cooling event near the end of the Eocene (Sancay, in progress; Yancey et al., in press).
Pseudolaesopollis ventosus Assemblage Zone
The Pseudolaesopollis ventosus Assemblage Zone is characterized by the occurrence of P. ventosus and Rhizophora spp. Both range below and above the Pseudolaesopollis ventosus Assemblage Zone, but they are the characteristic taxa for this interval. In this zone, however, Bombacacidites spp., Gothanipollis spp., Plagianthus? spp., and Nypa spp. are absent due to the cooling paleoclimate at the end of the Eocene. This is an interval zone between the late late Eocene Bombacacidites Assemblage Zone and the early Oligocene Hypoxylonites Assemblage Zone. Pseudolaesopollis ventosus occurs throughout the section and is suspected to be an ancestral mangrove, therefore its occurrence in dominantly marine and marginally marine paleoenvironments is expected.
EARLY OLIGOCENE (RUPELIAN)
Hypoxylonites Assemblage Zone
The Hypoxylonites Assemblage Zone is named for common occurrences of Hypoxylonites spp. in the lower Catahoula Formation. Other palynomorphs characteristic of the Hypoxylonites Assemblage Zone include Quercus spp., Rhizophora spp., Exesisporites neogenicus, and Kallosphaeridium biornatum. Exesisporites neogenicus is not seen below the Hypoxylonites Assemblage Zone, suggesting the potential for another FAD in the section. Species of Hypoxylonites occur earlier in the Eocene, but are common only from the early Oligocene to Recent (e.g., Elsik, 1969). Spores of this form genus are produced by extant Xylariaceae (Elsik, 1990), fungi which are ubiquitous wood destroyers in the modern habitat.
The 15 datasets from the GCPDB used in this study are centered on documentation of the Brazos River Valley section (Yancey, compilation in progress; Yancey et al., in press). Stratigraphic intervals (Text-Figure 3⇑), number of samples, and sample sites are summarized below:
Stone City Bluff - Stone City Member of the Crockett Formation - 19 samples. Type section of the Stone City Member exposed along the southwest bank of the Brazos River in Burleson County.
Stone City core - Stone City and Wheelock members of the Crockett Formation - 28 samples (McMahon, 1997). A 26 m core drilled close to the type section of the Stone City Member in Burleson County.
Rocky Branch - Wheelock Member of the Crockett Formation - 7 samples. Exposures near Stone City Bluff, Burleson County.
Little Brazos River - Wheelock Member of the Crockett Formation - 2 samples. East bank of Little Brazos River upstream from Highway 21 bridge, Brazos County. The samples are above a bentonite bed dated at 41.85 Ma.
Alabama Ferry landing - Wheelock Member of the Crockett Formation - 2 samples. Exposures downstream along the banks of the Trinity River, Houston County. The samples are bracketed by bentonite beds dated at 41.66 Ma and 41.53 Ma.
Hurricane Bayou - Hurricane Lentil of the Crockett Formation - 3 samples. Exposures in the type locality of the Hurricane Lentil near Crockett, Houston County.
Turkey Creek - Bryan Member of the Yegua Formation - 10 samples. Exposures on the west bank of Turkey Creek downstream from Highway 46 crossing west of Bryan and College Station, Brazos County.
Koppe Bridge - top of Yegua Formation including the Rock Prairie Member and base of the Caddell Formation - 17 samples. One sample from the bottom of Hopes Creek and 16 samples from exposures along the east bank of the Brazos River south of College Station, Brazos County.
Greens Prairie core - upper part of the Caddell Formation - 20 samples. Core drilled beside Greens Prairie Road interchange on Highway 6, south edge of College Station, Brazos County. Preliminary palynology by Yancey and Elsik (1994).
Gibbons Creek mine - upper Manning Formation - 56 samples (Gennett, 1996). Exposures and core in the Gibbons Creek lignite mine, east of the Navasota River and south of Carlos, Grimes County. The lowest samples are underlain by a bentonite bed dated at 34.53 Ma.
Lake Somerville dam - upper Manning Formation - 4 samples. Exposures on the south bank of the Lake Somerville dam outlet channel, Yegua Creek, Washington County.
Lake Somerville core - upper Manning Formation - 46 samples (Sancay, in progress). Core drilled on the south slope of the Lake Somerville spillway channel, Washington County.
Lake Somerville spillway - upper Manning Formation - 36 samples (Raymond et al., 1997). Exposures on the south slope of the Lake Somerville spillway channel, Washington County.
Riverside new quarry - lower Catahoula Formation - 1 sample. Exposure in quarry near FR 274 north of Huntsville, Walker County.
Riverside quarry - lower Catahoula Formation - 2 samples. Exposures in the old Riverside quarry, now Blue Lagoon diving park, north side of Huntsville, Walker County.
We thank J. A. Gennett for sharing her many contributions to the palynology of the late Eocene of Texas. Richard P. Zingula supplied samples from several localities in the Brazos River Valley and adjoining areas. The senior author expresses gratitude to his former employer, Exxon Company, U.S.A., for processing many of the field trip samples. This paper is one of a continuing series of contributions made possible by the Gulf Coast PalynoDataBank at The MycoStrat Connection.