Stratigraphic Framework of a Late Pleistocene Shelf-Edge Delta, Northeast Gulf of Mexico

Abstract

Lithologic, biostratigraphic, and chronostratigraphic data from a 92-m continuous surface boring, in the Main Pass area of the outer Mississippi-Alabama shelf, were used to calibrate high-resolution seismic profiles in a study of a late Pleistocene shelf-edge delta. The boring is the first of its kind through a shelf-edge clinoform wedge and the first ground-truth confirmation that the clinoforms in the study area are deltaic in origin. Chronologic control for the late Pleistocene outer shelf stratigraphy is based on the identification of Ericson Zones X, Y, and Z (alternating warm and cold water planktonic foraminifera zones) in the boring, representing at least the last 130 k.y. During sea level lowering related to the previous glacial maximum, the delta system progra ed onto a carbonate-rich outer shelf and upper slope starved of terrigenous sediments. The resulting seaward-thickening deltaic wedge is more than 100 m thick in localized areas. Stacking patterns of delta lobes demonstrate overall progradation from the northeast to the southwest. The ancestral Mobile River, possibly joined by the Pascagoula River, was the fluvial feeder of the shelf-edge delta. The upper portion of the delta wedge is extensively eroded, primarily by a broad swath of significant fluvial scour centered along the northeast- to southwest-oriented dip axis of the delta, and to a lesser extent by subsequent transgressive truncation. Fluvial scour resulted in a broad erosional trough. Evidence from the boring indicates that the erosional trough is filled with fluvial and estua ine facies. Thin estuarine and overlying marine units reflect transgression of the Lagniappe delta during the late Pleistocene--early Holocene transgression. According to standard sequence stratigraphic definitions and nomenclature, the extent of the sequence boundary, identified as the erosional base of the fluvial facies, places the majority of the outer shelf delta in the systems tract. Although the sequence stratigraphic approach is strictly a methodology, the term highstand is misleading for this systems tract. The portion of the delta thus categorized was built during the falling to lowstand minimum part of the relative sea level curve. Deposits of the actual in sea level that preceded progradation to the shelf edge occur instead on the inner and middle shelf s thin (<10 m) erosional remnants. Although eustatic sea level fall was large and rapid (at least 60 m in 20 ka), fluvial incision of the shelf break did not occur. Incision is instead best developed on the inner to middle shelf (about 20 m of erosional relief). No canyon or channel features occur on the upper slope that can be associated with this shelf-edge delta.