In search of the cause of catastrophic coastal change in Texas - West Louisiana Bay Evolution:
A study of Sabine and Calcasieu coastal plain systems
Kristy T. Milliken
Earth Science Department
This study is a culmination of a long-term investigation of the response of Texas and western Louisiana bays to changes in sea level and climate during the Holocene (last 10,000 years). The evolution of several systems (Calcasieu and Sabine, K. Milliken; Matagorda, J.K. Maddox,; and Nueces, A.R. Simms) are being extensively studied with a similar methodology to that utilized for the investigation of Galveston Bay evolution (Smyth, 1991; Thomas, 1992; Rodriguez et. al., in press). The work finished in Galveston provides a general stratigraphic framework coupled with a detailed radiocarbon chronology. Every system displays large scale backstepping associated with the eustatic sea level rise following the last glacial maximum ~18,000 cal BP. Sea level reached the inner shelf and the study area at ~ 10,000 cal BP. Superimposed upon the large scale backstepping facies are various periods of relative stillstand and even sediment progradation when sediment supply was great enough to keep pace or overwhelm sea level rise. Additionally, initial investigations by Smyth (1991) recognized that the bay/fluvial system has experienced small scale rapid changes at times when sea level was rising slowly (see also Thomas and Anderson, 1992). Subsequent work has determined that landward shifts in middle bay and bayhead delta facies of up to a kilometer/decade occurred. Rodriguez et. al., (in press) attributed these shifts to a combination of flooding lateral terraces and sediment supply (climate) variations. These progradational and/or retrogradational events occur at different times in different bays and potentially record a dramatic change in the sediment yield of different rivers associated with Holocene climate changes in the western onshore Gulf of Mexico.
An important goal of my research is to utilize the Gulf of Mexico coastal plain fluvial/bay sedimentary record to obtain a relationship between climate and sediment supply for the Holocene. Sediment supply in fluvial systems is largely controlled by drainage basin geology and climate. Drainage basin geology includes internal factors including drainage basin area, relief, vegetative cover, and lithology. Climatic factors include precipitation, temperature, and discharge. Although the geology and climatic conditions of an individual drainage system are closely linked and difficult to separate, I aim to extract the sediment supply signal by contrasting several systems. I have chosen a series of low relief drainage systems that span a precipitation/climate gradient, represent a series of drainage areas, and flow through similar sedimentary substrates (unlithified clastic sediments). The Gulf of Mexico coastal plain river systems generally trend N-S and span a sub humid to humid climatic gradient that follows the longitudinal trend. This potentially sets up an ideal situation in which, over time, the humidity/aridity zones shift E-W across the region of study. This thus represents an ideal natural laboratory to elucidate the relative effects of climate and geology on sediment supply.
Moreover, these small coastal plain rivers have not filled present day accommodation space and are currently marked at their marine limits by bays/estuaries. It is in this coastal zone where marine and fluvial depositional processes coincide. Utilizing these aspects, we can measure and quantify the relative affects of fluvial deposition compared to marine sedimentation in a given area over time. This potentially provides a measure of sediment supply variation. But, other factors possibly affect bay evolution and need to be constrained before a climate record can be determined from sediment supply variations. Factors controlling bay evolution include eustasy, antecedent topography, subsidence, and sediment supply. In order to understand the evolution of the fluvial/bay system it is necessary to attribute the resultant depositional environment (fluvial, delta, bay, or open marine) to the appropriate controlling mechanism. Antecedent topography and subsidence are accounted for by a basal surface (sequence boundary) map, regional correlations, and radiocarbon control. Regional sea level curves can be used to constrain the relative effects of eustasy. In order to understand the effects of sediment supply or climate variations at least two coastal river and/or bay systems from different climate systems must be contrasted. Using one system as the 'baseline', the differences in the other system can be attributed to relatively high or low sediment supply.
Furthermore, natural sediment flux of fluvial systems is difficult to constrain for long time periods. Historical records provide at best a century of data and are frought with complications such as sediment ponding behind dams and agricultural affects on natural landscapes and sediment erosion. The Holocene sedimentation record of small coastalplain fluvial systems trapped in bays provides a valuable record of accumulation rates over longer time scales. These can be compared to modern flux rates to investigate athropogenic impacts on drainage basin erosion and sediment flux rates.
Detailed facies mapping and chronostratigraphy control provide the framework in which to address multiple scientific and social questions including: 1) how low gradient coastlines will respond to future sea level and climate change; 2) does river discharge directly translate to sediment yield and how do high frequency (millennial to century scale) climate changes regulate sediment flux; 3) do geomorphic thresholds inherent in fluvial/bay systems impart a catastrophic reaction under constantly rising sea level and/or sediment supply reductions; 4) what is the nature and volume of sediments that are temporarily stored in incised valleys during trangressions, and how might this impact petroleum reservoir potential?