Age of Sediments Encasing Allochthonous Salt in the Gulf of Mexico; Clues to Emplacement History

Richard E. Kilby¹, F. A. Diegel¹, and M. J. Styzen^2
1Shell Exploration and Production Company, Houston, TX
²Shell International Exploration and Production, Houston, TX

Reflection seismic data indicate intact likely Mesozoic through Miocene strata in isolated blocks above allochthonous salt near the Sigsbee Escarpment. These blocks (Mesozoic “chips”) are bounded by unconformities and structural contacts with allochthonous salt. The stratigraphic section in the chips appears to be repeated at depth below the allochthonous salt indicating lateral transport along with the salt from the north. Identified chips range from 5 to 25 km in breadth.

The observed chips may contain stratigraphy similar to that reported from the GB-754 Norton Prospect, where Mesozoic and Paleogene strata are not highly condensed, but likely Miocene section is thin and cut by disconformities indicating seafloor expression of the underlying salt highs at that time.

Biostratigraphy from numerous well penetrations away from the chips shows that allochthonous salt is commonly covered by a carapace of condensed section, often less than 500 feet thick, and generally no older than Eocene. Subsalt wells commonly show mixed or inverted section of about the same age as the carapace and less than 1000’ thick. Older fauna generally drop out down-hole until a normal thickness, rightside-up section is encountered. The carapace is hemipelagic cover deposited on paleobathymetric highs above allochthonous salt. The mixed-or-inverted section, beneath, likely represents redeposited, eroded carapace overridden by salt advance.

The allochthonous Mesozoic chips are unlike the typical carapace as they contain the older stratigraphy deposited above autochthonous salt. Palinspastic reconstruction and regional sub-salt mapping identify candidate source locations for the chips in areas that, today, contain either very thick salt or younger, mainly Plio-Pleistocene strata in drop-in basins seperated by salt welds from older sub-salt section.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas