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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Depositional and Structural Settings
in Southwestern Louisiana
By
Lithologic and paleontologic variations within the Oligo-Miocene strata of southwestern Louisiana suggest a time of
highly fluctuating relative sea level, such that transgressive-regressive
cycles occurred in three orders of magnitude. The
second and third order cycles probably represent depocenter
shifts and glacial eustatic sea level fluctuations, while the first
order pattern reflects geotectonic and geosynclinal fill-compaction
relationships.
Within this cyclic regime, migration of the massive sandstone
facies, alternating sandstone and shale facies and
massive shale facies of southwestern Louisiana, produce
rather complex lateral facies relationships. To facilitate environmental
reconstruction within this system, a new coefficient
termed the facies index is introduced. The usefulness of this
coefficient, expressed by the equation:
Facies Index = net sand/(number of sands x shale thickness)
is based on the concept that a more proximal location to a
fluvio-deltaic depocenter
is not only characterized by higher
percentage sandstone but also by less sandstone-shale interbeddedness
for a given percentage sandstone (resulting in a
higher facies index).
By superimposing facies index maps on structure and isopach maps, contemporaneous structural and depositional frameworks are derived. More localized depositional and structural models, which are essential in establishing the hydrocarbon source and migration pattern as well as the reservoir characteristics and trapping mechanisms, can then be reconstructed as natural components of the regional system.
During the time of the Heterostagina and lower Discorbis zones of the Anahuac Formation (Oligocene), regressive east-west oriented delta front sands fluctuated (third order cycles) within an overall transgressive regime (second order). These deposits were then overlain by an extensive progradational fluvio-deltaic system which is represented by the upper Discorbis zone and lowermost Fleming Formation (Miocene). This regression was then interrupted by a large transgression which deposited a shale wedge that contained Siphonina davisi (intermediate neritic), Planulina palmerae and the upper bathyal Abbeville Assemblage (southern portion of study area). This transgression was followed by another regression and growth fault episode which prograded the shoreline to the southern extreme of the study area (Rockefeller Refuge of eastern Cameron Parish), where extremely thick lower Miocene sands were deposited within a structural embayment.
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