Intertidal benthic foraminiferal biofacies on the central Gulf Coast of Texas; modern distribution and application to sea level reconstruction

Six foraminiferal biofacies are defined for three tidal marshes on the central Gulf Coast of Texas. The Aransas River estuary and Port Bay marshes each contain four biofacies which occupy distinct elevational zones. Zonal definition at the Mustang Island marsh is less distinct due to the dominance of one biofacies. Analysis of core samples from the Port Bay marsh site enables reconstruction of Late Holocene sea levels and sedimentation rates. The resulting relative sea level curve contains evidence of a Late Holocene stillstand or highstand and shows that Late Holocene sea level rise continued to the present at this location. INTRODUCTION The intertidal zone is a delicately balanced natural system; erosion, deposition, wave action, tidal currents, topography, salinity variations within bays and estuaries, frequency and duration of tidal inundation, plant stabilization of sediments and a myriad of other critical factors all combine to produce a wide variety of intertidal sedimentary environments. The character of these environments, including flora, fauna and the sediment texture of the substrate, reflects this complex interplay of marine and terrestrial conditions. As a result, perhaps more than any other suite of depositional environments, intertidal environments are zoned narrowly according to sea level position (i.e. time of exposure in intertidal areas) and sediment characteristics often contrast greatly within a relatively small range of elevation. Given this delicate balance between elevation and sedimentary environments, the paleoenvironmental interpretation of intertidal deposits incorporated into the geologic record provides a means to track, with relative precision, former sea level positions. A common component of intertidal zones worldwide, benthic foraminifera have also been shown to occupy relatively narrow elevational zones in tidal marsh environments (Scott 1976; Scott and Medioli 1980a; Patterson 1990). Paleoenvironmental interpretation based on foraminiferal biofacies has proven to offer a high degree of elevational resolution relative to mean sea level, enabling deposits of low, middle and high intertidal environments to be distinguished in drill cores (e.g. Scott 1977; Scott and Medioli 1978; Scott et al. 1981; Smith et al. 1984; Williams 1989). The purpose of this study is to document the foraminiferal fauna of three tidal marshes, with contrasting salinity regimes, on the central Gulf Coast of Texas, and to define characteristic biofacies. The biofacies will be used to interpret depositional environments of core samples obtained from one of the marshes, in order to reconstruct local relative sea level history and the record of changing marsh sedimentation. Broad patterns of regional Holocene sea level changes have been established for many parts of the world (Clark and Lingle 1979), but in many areas detail is lacking, especially for the latter part of the Holocene Epoch, when the rates of sea level rise were substantially reduced from their postglacial maxima (Curray 1965; Kenney 1964; Hopkins 1967). In many areas, historical data, such as maps, air photographs and tide records, have been used to document recent sea level changes; however, uncertainty over long-term sea level behavior and the effects of human modification of the coastal zone makes it difficult to distinguish between historical and long-term sea level trends (Paine and Morton 1989). Biofacies analysis offers high resolution of former sea levels and has the potential to provide greater detail of sea level behavior than is presently available from sea level curves based on radiocarbon dating of nearshore deposits (Curray 1960; Nelson and Bray 1970). Uncertainty still exists over sea level changes along the Texas Gulf Coast, particularly during the Late Holocene (Shepard 1960); some researchers have suggested that sea level has been stable at its present position for several thousand years (Fisk 1944; LeBlanc and Bernard 1954; Fisk and McFarlan 1955; Gould and McFarlan 1959), while others have raised the possibility that sea level may have risen steadily and continuously over the same period (Shepard and Moore 1960). The issue is further complicated by inter-regional variations in tectonism and coastal subsidence: the Mississippi Delta region, for example, is undergoing subsidence due to crustal downwarping at a rate of about 0.3m per century (Akers and Holck 1957; Shepard 1960); the site of the present study, on the other hand, is believed to be relatively stable due to the underlying San Marcos Arch (Morton and McGowan 1980), therefore sea level history in this area should be comparable to that of other stable regions. Finally, the nature of sea level rise itself, whether gradual or episodic, has also been questioned (Anderson et al. 1991). Given recent concerns over the impact of global warming on sea levels (Barth and Titus 1984; Gornitz, et al. 1982), studies such as this are more relevant than ever before; a detailed Late Holocene sea level curve provides a baseline against which recent anthropogenic deviations from the long-term trend can be assessed and from which predictions of future sea level changes can be made. PREVIOUS WORK There has been a number of studies of foraminiferal distribution patterns in the marshes and bays along the coast of the Gulf of Mexico. Most of these studies have been surveys of the species present in broadly-defined coastal environments, including marshes, bays and open coastal waters. Although species characmicropaleontology, vol. 40, no. 2, pp. 169-183, text-figures 1-7, tables 1-3, 1994 169 This content downloaded from 157.55.39.120 on Mon, 05 Sep 2016 04:09:50 UTC All use subject to http://about.jstor.org/terms H. F L. Williams: Intertidal benthicforaminiferal biofacies, Central Gulf Coast of Texas TEXT-FIGURE 1 Location map showing the three marsh sites, sample sites along each transect and core site location. teristic of particular environments have been identified, none of these previous studies have quantitatively defined small scale elevational zones occupied by distinct foraminiferal biofacies, or attempted to apply foraminiferal biofacies analysis to the interpretation of subsurface deposits. Korfeld (1931) examined 27 samples collected from the lee of barrier islands, embayments and shallow open coastal areas along the Texas and Louisiana coast. In the vicinity of collection sites used in the present study (Corpus Christi Bay and Copano Bay, Texas), 13 species were identified: Ammobaculites cf. foliaceum (Brady); Elphidium discoidale (d'Orbigny); Elphidium gunteri C le galvestonense Korfeld; Elphidium incertum (Williamson) m xicanum Kornfeld; Massilina peruviana (d'Orbigny); Nonion depressula (Walker and Jacob) matagordana Korfeld; Quinqueloculina auberiana d'Orbigny; Quinqueloculina costata d'Orbigny; Quinqueloculina seminula (Linne); Rotalia beccarii (Linne) parkinsoniana (d'Orbigny); Rotalia beccarii (Linne) tepida Cushman; Spirillina vivapara Ehrenberg; Triloculina oblonga (Montagu). Sampling of marsh environments did not form