294-7 Matagorda Formation: A New Holocene Coastal Stratigraphic Unit, Texas

Session: Geologic Mapping (Posters)

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Abstract:

The Matagorda Formation is a newly proposed stratigraphic unit that comprises Holocene coastal (sea-level influenced) deposits on the Texas coastal plain. This formation consists of sediments deposited in fluvial-deltaic, bay and bay-margin, and Gulf of America (Gulf of Mexico)-margin paralic environments since the Last Glacial Maximum (marine oxygen isotope stage 2, or MIS 2) within and at the margins of stream valleys incised into the coastal plain since the last Pleistocene sea-level highstand during MIS 5. The Matagorda Formation includes informal coastal facies that are influenced by sea level and grouped into interfingering fluvial-deltaic deposits (Lavaca Member), bay and bay-margin deposits (Carancahua Member), and paralic Gulf-margin deposits (Saluria Member). Other coastal facies, such as eolian sand or silt sheets and clay dunes, are included in the Matagorda Formation but are not included within the members. The type area is the Matagorda Bay system on the central Texas coast, where many examples of facies within the proposed formation are exposed along and near the shoreline. The upper boundary of the Matagorda Formation is the active depositional or erosional surface either on land or at the water bottom beneath coastal streams, tidal channels, and bays. The lower boundary is a composite unconformity that may be marked by (1) a mature soil or erosional surface at the top of the Pleistocene Beaumont Formation (a sharp boundary with or without soil development below the unconformity), (2) the top of alluvial terrace deposits above the Beaumont Formation (Deweyville Formation or broad equivalent) within the incised valley (sharp boundary with or without soil development below the unconformity), or (3) fluvial deposits above the Beaumont Formation at the base of the incised valley that predate the Holocene transgression (possibly a gradational boundary where active deposition was occurring as sea level rose and base level reached the incised valley).

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9820

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