ABSTRACT: Facies Architecture and Sequence Stratigraphy of the Late Quaternary Incised Valley Fill of the Mississippi River

PENLAND, SHEA, and RANDOLPH A. MCBRIDE, Louisiana Geological Survey, Baton Rouge, LA, JOHN R. SUTER, Exxon Production Research, Houston, TX, RON BOYD, Centre for Marine Geology, Dalhousie University, Halifax, Nova Scotia, Canada, and S. JEFFRESS WILLIAMS, U.S. Geological Survey, Reston, VA

The Late Quaternary depositional history of the sediments infilling the incised valley of the Mississippi River have been documented using vibracores, borehole records, deep borings, high-resolution seismic profiles, radiocarbon dates, and historical maps/photographs. During the late Wisconsinan lowstand, relative sea level stood +/- 125 m lower than present, fixing the position of the Mississippi fluvial/deltaic system at the mouth of its incised valley and the head of its submarine canyon, on the margin of the continental shelf off south-central Louisiana. At lowstand, the fluvial/deltaic system facies were predominantly sandy braided stream deposits confined to the Mississippi canyon and fan. Fisk and McFarlan (1955) termed these deposits the substratum. Melting of the Laurentide g aciers 18,000 years ago initiated the Holocene transgression that caused sea level to rise and force the Mississippi fluvial/deltaic system to retrograde into the confines of its incised valley. A series of backstepping shelf-phase delta plains developed during relative sea level stillstands, onlapping the surface of the substratum deposits. The character of the material coming down the Mississippi River changed from coarser grained sediments at the time of maximum late Wisconsinan deglaciation to finer grained sediments through the Holocene. Fisk and McFarlan (1955) termed the fine-grained deltaic deposits the topstratum. The first shelf-phase delta plain was established approximately 12,000-15,000 years ago when sea level stood 40-50 m lower than present. Additional shelf-phase delta p ains were deposited during the later stages of the Holocene transgression. The series of shelf-phase delta plains comprising the topstratum systematically decrease in thickness and overall grain-size upward through the valley fill sequence, reflecting a diminishing sediment supply and accommodation space following maximum sea level lowstand.

Within a sequence stratigraphic framework, the substratum deposits represent the lowstand systems tract of the Late Quaternary Mississippi fluvial/deltaic system. The topstratum deposits represent the transgressive systems tract, except for the uppermost shelf-phase delta plain, which represents the highstand systems tract, assuming the Holocene transgression ended 3000 years ago. Each of these shelf-phase delta plains represent a parasequence set tied to a particular sea level stillstand in the Holocene. The base of each parasequence set can be defined by offlapping prodelta clinoforms, distributary channeling, and/or tidal inlet channeling. The top of each parasequence set can be defined by a flooding surface, ravinement surface, and/or a condensed section.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)