ABSTRACT

Interpretations of vibracores from the Bayou Grand Caillou area form the database for delineating the facies architecture of an abandoned shallow water delta lobe within the Lafourche delta complex of the Mississippi River delta plain. The modern landscape of this shallow water delta is dominated by a network of distributary channels, which bifurcate and isolate smaller portions of the delta plain into small-scale interdistributary basins characterized by extensive swamps that occur at the apex of these basins and grade seaward into fresh, intermediate, and saline marshes.

The Bayou Grand Caillou distributary network, which consists of Bayou Grand Caillou, Bayou Chauvin, Four Point Bayou, and Bayou Sale, was the third delta lobe of the Lafourche complex to build. This distributary network prograded seaward between Bayou du Large to the west and Bayou Petit Caillou to the east less than 2000 years B.P. With delta abandonment, shoreface erosion reworked the Bayou Grand Caillou distributaries, generating the Isles Dernieres barrier island arc over approximately the last 600 years. This protective transgressive barrier shoreline forms the seaward geologic framework of the Bayou Grand Caillou deltaic estuarine system.

The average thickness of the shallow water delta is <10 meters. The regressive component of the delta generally averages 7-8 meters thick, increasing to as much as 15 m thick in areas of distributary channelling. The regressive component coarsens upward and consists of prodelta mud at the base overlain by outer and inner fringe silty sand deposits. Typically, this sequence is capped by fine-grained organic-rich (>35%) marsh deposits. The distributary channel deposits are characterized by a sharp erosional contact at the base overlain by a clean fine-grained sand that is normally characterized by a subtle fining-upward sequence.

These shallow water delta deposits, which are bound by transgressive surfaces of erosion, form a parasequence. The base of the parasequence is characterized by an erosional unconformity interpreted as a ravinement surface produced by shoreface erosion. The top of the parasequence is characterized by a marine flooding surface produced by transgressive submergence as a function of deltaic abandonment, eustatic sea level rise (0.23 mm/yr), and regional subsidence (1 cm/yr). A parasequence is the building block of a systems tract. Overall, the Bayou Grand Caillou is part of the highstand systems tract of the modern Mississippi River delta plain.