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ABSTRACT: Lower Miocene-Early Pliocene Deposystems in the Gulf of Mexico: Regional Reservoir System Architecture

FILLON, RICHARD H., Earth Studies Associates, New Orleans, LA; and PAUL N. LAWLESS, CNG, Inc., New Orleans, LA

Isopach and net-sand maps drawn from a dataset of thousands of wells clarify third-order onlap-offlap relationships within the Miocene and lower Pliocene of the northern Gulf. Two second-order "supersequences", one in the lower Miocene, bracketed by the Lentic jeff and Big hum sequence boundaries, and one in the middle Miocene-lower Pliocene, bracketed by the Big hum and Ceratolithus acutus sequence boundaries, contain respectively, six, and seven, third-order sequences. At the super-sequence level accumulation rate patterns reflect depositional trends related to global changes in continental erosion, while at the third-order level deposystem patterns reflect local changes. Sand is nearly equally apportioned between the Louisiana and Texas shelves within lower Miocene third-order sequences and unequally distributed within middle Miocene-lower Pliocene sequences. Beginning in the middle Miocene, significantly more sand and other sediment was delivered to the northern Gulf by the Mississippi River than by the combined Texas rivers. This dramatic change may reflect the capture of a large drainage system, e.g., the proto-Red River system, by the Mississippi. Sand deposition on the Texas shelf was further reduced during the imputed sea level rise portion of the middle Miocene-lower Pliocene super-sequence, perhaps in response to increasing aridity. Sea level fall portions of both super-sequences produced small shelf-edge sand depopods delivering more sediment to the slope and abyssal plain. Sand depopods in Louisiana anastomosed, varied considerably in size, and generally onlapped in the west and offlapped in the east. In Texas, sand was deposited in coast-parallel depopods that varied less in size and alternately onlapped and offlapped the margin.

FILLON, RICHARD H.,  and PAUL N. LAWLESS

 

AAPG Search and Discovery Article #90908©2000 GCAGS, Houston, Texas