Sequence Stratigraphy and Facies Variations within Neogene Turbidite Systems, Northern Deep Gulf of Mexico

WEIMER, PAUL, University of Colorado, Boulder, CO

The Neogene strata of the northern deep Gulf of Mexico are characterized by turbidite systems of differing stages of evolution and depositional setting. These Neogene sequences are the basinal, downdip equivalent strata to the petroleum-producing intervals in the continental shelf and slope to the north and northwest. The middle to late Miocene Upper Mexican Ridges sequence (12.5-5.5 Ma) and Cinco de Mayo sequence (5.5-4.5 Ma) are interpreted to consist primarily of unchannelized to partially channelized basinal turbidite systems. South of the eastern portion of the Mississippi Fan foldbelt, these sequences are characterized by a series of subparallel, hummocky, and bidirectionally downlapping mounded reflections, with high to moderate amplitude and fair to moderate continuity. Many s all cut-and-fill features are present in the sequences; small channel-levee systems may be present only in a few places. These two sequences are coeval to the development of the Mississippi Fan foldbelt. Wedges of strata that become thinner to the south indicate that the foldbelt contributed minor amounts of locally derived sediment to these sequences. The overlying Mississippi Fan (4.5 Ma to present) consists of 17 seismic sequences that have a much larger areal distribution than the two underlying Miocene sequences. Each fan sequence is characterized primarily by sediments deposited in channel, levee, and overbank settings. Variations within the sequences have been documented in regard to the evolution of the channels, mass transport-related material deposited at the base of most seque ces, and vertical change in the amplitude of the levee reflections (and possibly grain size) throughout any sequence.

The differences among these turbidite sequences are caused by both the overall progradation of the continental margin and differences in the magnitude of sea-level fluctuations and volumes of sediment deposited in the deep Gulf of Mexico during the Neogene. The Upper Mexican Ridges and Cinco de Mayo sequences are interpreted to be the first coarse-grained turbidite systems deposited in this area of the deep Gulf of Mexico, with the overlying Mississippi Fan being part of a continuum in deep-water depositional systems. The lack of significant channel-levee systems suggests that the Upper Mexican Ridges and Cinco de Mayo sequences contain basinal, unchannelized facies in contrast to the equivalent updip strata to the north. The oldest sequences in the Mississippi Fan were relatively sma l and approximately the same size as the Upper Mexican Ridges and Cinco de Mayo sequences. The coeval Miocene deltaic depocenter on the shelf to the north-northwest had not migrated as far to the south as during the Pleistocene, limiting the volume of sediments deposited in the deep Gulf of Mexico. The magnitude of sea-level fluctuations, which controlled the timing of deep-water sedimentation, was not as large during the middle to late Miocene as during the Plio-Pleistocene. In addition, the depositional rates within the Mississippi Fan sequences were considerably higher than the rates in the Upper Mexican Ridges and Cinco de Mayo sequences.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)