Composition and Morphology of Late Quaternary Carbonate Deposits, Outer Continental Shelf, Northeast Gulf of Mexico

W. W. Schroeder, D. J. Benson, P. J. Pearce, K. M. Hancock, W. W. Sager, R. O. Howard, and A. W. Shultz

Organically-produced carbonate features are common on the outer continental shelf from east of the Mississippi River delta to the western rim of the De Soto Canyon. The features are composed of various assemblages of calcareous algae, cheilostome bryozoans, serpulid worms, foraminifera, and corals. Lithologies are primarily bindstones with some associated wackestones and packstones. Marine and some possible meteoric cements are minor components.

The features investigated to date occur in five morphologic configurations: pinnacles, flat-top reefs, patch reefs, reef-like mounds, and isobath parallel ridges. Pinnacles are high-relief, spire-like structures 10-50 m wide at their bases and up to 18 m tall. They are found in an elongated, curved cluster in the southwest part of the study area at depths of 105-120 m and scattered in the far west at depths of 77-90 m. Flat-top reefs are broad, steep sided-features up to 1000 m across and 15 m in vertical relief. They are located in the west-central region between 74-82 m. Patch reefs are mostly mushroom shaped with 1-5 m wide pedestal-like bases and bulbous tops up to 10 m across. They occur at depths of 74-84 m in at least two separate fields in the western region. Reef-like mounds re found along the western rim of the De Soto Canyon. They are 10-70 m wide, up to 4 m high and are found at depths of 70-80 m. Isobath parallel ridges are 10's to 100's of meters wide and up to 15 km long with seaward facing escarpments up to 8 m in relief. Most are confined to a depth range of 68-76 m. The carbonate material represents a crust over relict barrier island or longshore bar deposits.

These organic features developed during the transgression following the last Wisconsinan lowstand. Subsequent growth accompanied sea level rise until they were either unable to keep pace with the rate of rise and drowned and/or were exposed to regional environmental changes, such as variations in magnitude or location of fresh water and sediment discharge, that were unsuitable for normal growth and reproduction.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California