Taylor E. Blood and Dennis W. Cratsley
Minerals Management Service, 1201 Elmwood Park Boulevard, New Orleans, Louisiana 70123
e:mail taylor_blood@smtp.mms.gov

Abstract

The lower Miocene, the Burdigalian and Aquitanian Stages, has previously been interpreted separately Vail et al. (1977) and Haq et al. (1988) as being deposited in four third order sequences. The boundaries of the lower Miocene occur just above the sequence boundaries at 25.5 and 16.5 million years ago. Efforts at the Minerals Management Service to construct a stratigraphic framework have resulted in the inclusion of four previously unrecognized Type I sequence boundaries. All the additional sequences are in the Aquitanian portion of the lower Miocene. The age dating of these sequences has ben based mostly on benthic paleontologic data, as little planktonic information has been provided to the Minerals Management Service.

Prentice and Matthews (1988),using data on oxygen isotopes of deep-water benthic verses planktonic foraminifera, drew a glacioeustatic curve for the period from 70 million years ago to the present. This curve shows that the sea-level, based on ice volumes, was between 80 and 160 meters below the present level and each rise of fall in the lower Miocene was 20 to 40 meters. Based on the Milankovitch-band insolation changes a theoretical sea-level curve for any 5 million year span of the Tertiary was produced, Prentice and Matthews (1991). This curve shows 6 sea-level falls on the order of 50 to 100 meters in a 5 million year period. Ye et al. (1993( put forth evidence that the lower Miocene sedimentation has been affected by even the highest frequencies of the Milankovitch-band insolation changes. In this paper the authors interpret that there are falls in sea-level in the range of 20 to 40 meters at time intervals averaging 0.8 million years. These values for sea-level fluctuations are the same as anticipated from the Matthews and Frolich (1991) glacioeustatic curve based on oxygen isotopes.

The magnitude of sea-level fluctuations for the lower Miocene, interpreted by us to have caused Type I sequence boundaries, is on the order of 50 to 100 meters which corresponds closely with the theoretical sea-level changes based on insolation calculated by Prentice and Matthews (1991). The magnitude of the rises and falls of sea-level creating these Type I sequences is based on paleoecology data for marine shales, depositional modeling for sands, seismic facies, and sequence studies. These data are combined by constructing geologic cross sections at both the regional and field level. The seismic and geologic sections shown are from Mustang, Matagorda, High Island, ans West and East Cameron Areas of Federal waters.

Downthrown to the regional growth faults, middle to outer neritic shales have been incised by valleys in which were deposited fluvial and deltaic point bars. Upthrown to the growth faults in Western Mustang Island Area, the seismic facies is characterized by strong parallel reflectors, and only thin sands were deposited. This was interpreted as transgressive and highstand deposits of thin sands.

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