Jet-Plume Depositional Bodies—The Primary Building Blocks of Wax Lake Delta

Robert Wellner¹, Rick Beaubouef², John Van Wagoner³, Harry Roberts⁴, and Tao Sun^5
1ExxonMobil Upstream Research Company, P.O. Box 2189, Houston, Texas 77252-2189
²ExxonMobil Exploration Company, 223 Benmar, Houston, Texas 77060
³ExxonMobil Upstream Research Company, P.O. Box 2189, Houston, Texas 77252-2189
⁴Louisiana State University, Coastal Studies Institute, Baton Rouge, Louisiana 70803-7527
⁵ExxonMobil Upstream Research Company, P.O. Box 2189, Houston, Texas 77252-2189

Analysis of Wax Lake Delta, a modern bay head delta located in the western portion of the Mississippi River Delta Complex, indicates that this delta is comprised of morphologically similar depositional bodies. Each of the identified bodies has predictable and statistically similar rock property trends. This relationship is thought to occur because the bodies have the same causal mechanism: fully turbulent and expanding flow (wall jets) from a distributary channel.

Interpretation of high-resolution aerial photographs indicates Wax Lake Delta formed by progradation and compensational stacking of jet-plume deposits (bodies). The growth of an individual body occurs over months to years. In general, each of these bodies has a teardrop shaped, plan view morphology with an aspect ratio of 0.4-0.5. Core and log interpretation indicates that the head of the body is characterized by thick sand deposits (up to 3 m) that are typically trough cross-bedded and/or massive, and fill a scour pool. Vertical grain-size trends at any one location are typically uniform. The transition from the relatively confined scoured region to a conformable surface occurs over ~100 m and also marks the transition from bedload to suspended load deposition. The suspended load deposits form a radial apron seaward of the scour pool. This suspended load region fines and thins away from the terminal end of the scour and is characterized by bed-sets that exhibit partial or complete Bouma sequences.

Abandonment of a body begins when the deposits reach an elevation near mean low sea level. As this occurs, flow from the mouth of the distributary channel is forced into the topographic lows on either side of the stranded body. New bodies then develop adjacent to, and slightly seaward of, their stranded counterparts. In general, the readily available accommodation between bodies, not channel incision, is the dominant control on the development of new depocenters.

The development of Wax Lake Delta can be consistently interpreted using a turbulent jet flow model, which departs from the classic model of distributary channel, stream-mouth bar, delta front, and prodelta deposition. A new model, based on the compensational stacking of jet-plume depositional elements or bodies, is proposed herein. More globally, this model is currently being tested and modified using physic-based forward numeric models and field observations. Initial observations indicate that this model can help facilitate the development of comprehensive stratal architectures in the subsurface and outcrop.

AAPG Search and Discovery Article #90080©2005 GCAGS 55th Annual Convention, New Orleans, Louisiana