Stratigraphy and History of the Holocene Sediments in the Sabine-High Island Area, Gulf of Mexico

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Abstract
The Holocene sediments on the northwest shelf of the Gulf of Mexico were formed in interdeltaic as well as deltaic areas during the postglacial rise of the sea and during its stillstand since it reached its present level. In an interdeltaic area of relatively minor sedimentation off the Texas coast, the Sabine-High Island area, 11 lithotypes were defined during a three-dimensional study of the Holocene sediments. With the exception of one type, peat, all are made up of various combinations of three major sediment components, clay, quartz sand, and shells. These sediments were deposited as five lithofacies on an eroded Pleistocene surface that was crossed by a major topographic feature, the channel of the Sabine and Calcasieu Rivers. From bottom to top these lithofacies are: (1) fluvial or fluvial-deltaic sand that is confined to the channel; (2) peat which is also confined to the channel and which was deposited in an environment that transgressed up the channel as sea level rose; (3) muddy sediments which accumulated initially in a lagoon or estuary and later in the open marine environment as the rising sea flooded the river valleys; (4) clean quartz sand that was initially deposited along a shoreline and later in a shallow marine environment to form Heald and Sabine Banks, the major topographic features on the present-day sea floor; and (5) molluscan shell debris that accumulated in the marine environment primarily by the growth of organisms but also by reworking of shells from older sediments. Data developed in this study trace the Holocene history from 10,200 years B.P. to the present. Prior to this time fluvial sedimentation was filling the glacial stage river valleys. At 10,200 years B.P. the rising sea began to flood the Sabine valley and convert it into a lagoon or estuary. A swamp formed ahead of the rising water. At this time sea level was at —117 feet. The sea continued to rise rapidly until it reached a level of —73 feet at 9,400 years B.P. when the rate of rise decreased. For the next 2,800 years, until 6,600 years B.P., sea level fluctuated between —62 and —72 feet. Sand began to accumulate along a shoreline just south of the study area during this time. At 6,600 years B.P. sea level again began to rise rapidly; this continued until 5,650 years B.P. when the rate of rise decreased. Sea level reached its present elevation approximately 2,800 years ago. Important geologic events during this period were: (1) inundation of the upland, (2) the change from a brackish water to a marine environment in the vicinity of the banks (5,200± years B.P.), (3) accumulation of sand in the marine environment to form present-day Heald and Sabine Banks, and (4) establishment of the shoreline on the coastal plain and migration of part of this shoreline seaward as the Sabine estuary filled with sediments. The three most important factors controlling the distribution of sediments are considered to be topography, variations in the rate of rise of the sea, and currents. An approximate stillstand of the sea, with only minor fluctuations at a time when a ridge of the Beaumont Clay (Pleistocene) was exposed, led to concentration of the sand to form Sabine and Heald Banks. A longshore current brought the sand to this area; a current derived from the Sabine and Calcasieu Rivers is believed to have been a counter force that prevented widespread distribution of the sand over the sea floor. This volume was based on a symposium, Deltaic Sedimentation, which was held at the AAPG/SEPM Annual Meeting in New Orleans, Louisiana on April 1965. Many geologists have become involved in studies of deltaic sediments and sedimentation processes. Some of the papers in this volume are based on detailed local studies of modern deltaic sedimentary sequences, on processes of deposition, and on physical and biological characteristics of the deltaic environments.

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