Sequence Stratigraphy of a Coastal-Plain Incised Valley Estuary: Lake Calcasieu, Louisiana

Abstract

ABSTRACT The stratigraphic profile along a 100 km section of the Lake Calcasieu incised-valley fill was reconstructed from 87 bridge borehole records at four sites along the tidal reaches of the Calcasieu River, an additional 18 boreholes aligned along the chenier plain near the estuary mouth, and shallow seismic data collected along 160 km of track line within the Lake Calcasieu central basin. Valley incision occurred during the d18O stage 2 sea-level lowstand, when the paleo-Calcasieu River flowed across the inner continental shelf and incised into older fluvial terraces of the Prairie Formation. The valley floor is 57 m deep beneath the modern chenier plain and rises to -30 m along the upper reaches of the modern estuary. Sea.level rise commenced about 20 ka and has continued episodically throughout the Holocene. Shoreline transgression has proceeded at varying rates in response to sea-level rise, and the incised valley is now occupied by a wave-dominated estuary, fringed by the chenier plain of western Louisiana. Lowstand deposits are poorly preserved within the Calcasieu incised valley. Deposits constituting the transgressive systems tract (TST) lie directly above the lower sequence boundary at the top of the Prairie Formation. The TST shows greatest facies diversity and clearest separation of bounding surfaces toward the present seaward end of Lake Calcasieu. Here, the TST comprises three parasequences that include fluvial, bayhead-delta, and central.basin (estuarine) deposits. Multiple flooding surfaces within the TST relate to the episodic style of sea-level rise. Highstand infilling of the valley has been characterized by nearshore marine deposition and chenier-plain progradation. Along the lower reaches of the modern Calcasieu River, the valley fill comprises transgressive central-basin nd landward-stepping bayhead-delta facies, overlain by the highstand bayhead-delta complex. This study documents the most recent phase of deposition within the compound fill of a coastal-plain incised valley and demonstrates the potential importance of fluvial controls on facies within incised-valley systems.