Kansas Geological Survey, Subsurface Geology 12, p. 23-24
S. M. Greenlee and T. C. Moore
Exxon Production Research Company
Eustatic sea-level positions for the Middle-Late Miocene and Early Pliocene have been estimated through analysis of prograding siliciclastic sediments from the Baltimore Canyon trough and the northeastern Gulf of Mexico. Here we describe the Neogene stratigraphy of these areas, the methodology used to derive sea-level estimates, and the results obtained. A companion paper by Schroeder and Greenlee (this volume) discusses basin-modeling techniques that were used to model the observed stratigraphy of the area and test sea-level estimates.
The Middle and Upper Miocene and Lower Pliocene strata in both study areas consist of a series of lobate, progradational depositional sequences. Each sequence is composed of strata which onlap below the preexisting shelf margin, interpreted as low-stand deposits, and more areally extensive high-stand deposits. These sequences have been correlated, using available biostratigraphy from exploratory wells, to third-order eustatic cycles noted on the chart of Haq et al. (1987).
Sea level was calculated at each high-stand shelf edge using the formula
1.446SL = SEDLOAD + UNCONT + SUB + PWD - Z
(Hardenbol et al., 1981)
Using this formula, we are tracking sediment-accommodation potential through time and compensating for the effects of isostatic and tectonic subsidence. Tectonic subsidence is calculated using a method based on angular-rotation rates of the subsiding margin shown in fig. 1. This method is preferable to geohistory analysis in this case as no assumption of eustasy is incorporated. Our primary uncertainty in calculated sea levels, apart from the reliability of the stratigraphic-age determinations, relates to estimating the paleo water-depth at the shelf margin. Paleo water-depth estimates of benthic foraminifera and facies analysis have been used to constrain these factors. In addition, due to erosion during sea level or nondeposition during the highest portions of the third-order cycles, we may not be measuring the highest stands of sea level. We attempted to measure third-order sea-level falls by measuring the downward shifts in onlapping shallow-marine sediments from high-stand to low-stand positions.
Figure 1--Method used to calculate tectonic subsidence rate of continental margin (from Moore et al., 1987). An average angular-rotation rate from a calculated hinge point is determined based on basinward thickening of sediments. This rate is then used to derive a value of tectonic subsidence at the paleo-shelf edge relative to present sea level.
Our estimates of specific sea-level highs and lows are consistent to within about 20 m (66 ft) in the two study areas. High-stand sea-level estimates are lower than most previous studies. The amount of long-term sea-level fall during the Middle to Late Miocene calculated here is 24-42 m (79-139 ft). Third-order sea-level fluctuations of 12-116 m (40-383 ft) were estimated. The long-term sea-level fall is similar to that calculated by estimates of changes in oceanic ridge volume (Kominz, 1984) but lower than that depicted on the curve of Haq et al. (1987). Estimates of the magnitude of third-order fluctuations are similar to those represented on the Haq et al. (1987) curve.
Haq, B. U., Hardenbol, J., and Vail, P. R., 1987, Chronology of fluctuating sea levels since the Triassic: Science, v. 235, p. 1,156-1,167
Hardenbol, J., Vail, P. R., and Ferrer, J., 1981, Interpreting paleoenvironments, subsidence history, and sea-level changes on passive margins from seismic biostratigraphy--26th International Geological Congress, Geology of Continental Margins: Oceanologica Acta, Supplement, v. 4, p. 33-44
Kominz, M. A., 1984, Oceanic ridge volumes and sea-level change--an error analysis; in, Interregional Unconformities and Hydrocarbon Accumulation, J. B. Schlee (ed.): American Association of Petroleum Geologists, Memoir 36, p. 100-127
Moore, T. C., Loutit, T. S., and Greenlee, S. M., 1987, Estimating short-term changes in sea level: Paleooceanography, v. 2, p. 625-637
Kansas Geological Survey
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Web version May 7, 2010. Original publication date 1989.