Signatures of climate vs. sea-level change within incised valley-fill successions: Quaternary examples from the Texas GULF Coast

Abstract

The passive margin Texas Gulf of Mexico Coastal Plain consists of coalescing late Pleistocene to Holocene alluvial–deltaic plains constructed by a series of medium to large fluvial systems. Alluvial–deltaic plains consist of the Pleistocene Beaumont Formation, and post-Beaumont coastal plain incised valleys. A variety of mapping, outcrop, core, and geochronological data from the extrabasinal Colorado River and the basin-fringe Trinity River show that Beaumont and post-Beaumont strata consist of a series of coastal plain incised valley fills that represent 100 kyr climatic and glacio-eustatic cycles. Valley fills contain a complex alluvial architecture. Falling stage to lowstand systems tracts consist of multiple laterally amalgamated sandy channelbelts that reflect deposition within a valley that was incised below highstand alluvial plains, and extended across a subaerially-exposed shelf. The lower boundary to falling stage and lowstand units comprises a composite valley fill unconformity that is time-transgressive in both cross- and down-valley directions. Coastal plain incised valleys began to fill with transgression and highstand, and landward translation of the shoreline: paleosols that define the top of falling stage and lowstand channelbelts were progressively onlapped and buried by heterolithic sandy channelbelt, sandy and silty crevasse channel and splay, and muddy floodbasin strata. Transgressive to highstand facies-scale architecture reflects changes through time in dominant styles of avulsion, and follows a predictable succession through different stages of valley filling. Complete valley filling promoted avulsion and the large-scale relocation of valley axes before the next sea-level fall, such that successive 100 kyr valley fills show a distributary pattern. Basic elements within coastal plain valleys can be correlated with the record offshore, where cross-shelf valleys have been described from seismic data. Falling stage to lowstand channelbelts within coastal plain valleys were feeder systems for shelf-phase and shelf-margin deltas, respectively, and demonstrate that falling stage fluvial deposits are important valley fill components. Signatures of both upstream climate change vs. downstream sea-level controls are therefore interpreted to be present within incised valley fills. Signatures of climate change consist of the downstream continuity of major stratigraphic units and component facies, which extends from the mixed bedrock–alluvial valley of the eroding continental interior to the distal reaches, wherever that may be at the time. This continuity suggests the development of stratigraphic units and facies is strongly coupled to upstream controls on sediment supply and climate conditions within hinterland source regions. Signatures of sea-level change are critical as well: sea-level fall below the elevation of highstand depositional shoreline breaks results in channel incision and extension across the newly emergent shelf, which in turn results in partitioning of the 100 kyr coastal plain valleys. Moreover, deposits and key surfaces can be traced from continental interiors to the coastal plain, but there are downstream changes in geometric relations that correspond to the transition between the mixed bedrock–alluvial valley and the coastal plain incised valley. Channel incision and extension during sea-level fall and lowstand, with channel shortening and delta backstepping during transgression, controls the architecture of coastal plain and cross-shelf incised valley fills.