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PSA Sequence Stratigraphic Framework for the Lower Cretaceous North Carolina Coastal Plain, Southeastern U.S.A
By
Richard Sunde1 and Brian P. Coffey2
Search and Discovery Article #50044 (2007)
Posted June 18, 2007
*Adapted from poster presentation at AAPG Annual convention, Long Beach, California, April 1-4, 2007.
1Simon Fraser University, Burnaby, British Columbia, Canada ([email protected])
2Simon Fraser University, Vancouver, British Columbia, Canada ([email protected])
A lithology-based sequence stratigraphic framework for the little studied Lower Cretaceous mixed carbonate-siliciclastic sediments of the subsurface Albemarle Basin is presented. Thin sections produced of well cuttings from exploration wells were analyzed to characterize lithology, fossil components, depositional facies, and diagenetic events because the study interval is entirely confined to the deep subsurface in a basin lacking core control. Data was then used in conjunction with wireline logs to document the facies abundance and stacking patterns. Integration of 2D seismic data and biostratigraphic control allowed regional correlation of major transgressive-regressive events between wells, resulting in the generation of a sequence stratigraphic framework for the onshore basin.
Results show the following dominant lithofacies (listed from shallow to deep): sandstone, skeletal sandstone; variably sandy mollusk packstone/grainstone; siltstone; unfossiliferous and diatomaceous shales; skeletal wackestone; variably sandy (quartz and glaucony) lime mudstone, and marl.
Comparison of observed facies with cores and wireline logs from the Baltimore Canyon and Southeast Georgia Embayment confirms the sequences consist of upward-shoaling siliciclastic shorefaces, with basal-open shelf mollusk-rich carbonates often marking transgressive events. Basin-scale depositional trends indicate greater accumulation of the carbonate facies in the southern basin, with increased fine siliciclastic material to the north. This trend may reflect a major siliciclastic point-source in the vicinity of the ancestral Chesapeake region.
The depositional and diagenetic models generated from this research provide valuable insight into the facies and reservoir properties in coeval offshore units comprising frontier exploration targets along the Atlantic coast of the U.S. and Canada.
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The presence of wide, strike parallel facies belts suggests a high wave energy influence.
Two general stacking patterns have been recognized: Clastic-Dominated (usually the HST of the sequences): An early quartz-sandy, molluscan-skeletal packstone defines a transgressive lag. This is overlain by shales and siltstones grading up into sandstone representing a prograding shoreface. Bivalve (oyster) fragments are common, and the sequence is rarely capped by a coal layer. Carbonate-rich beds may represent winnowing of the sediment by tempestites or temporary clastic sedimentation hiatuses (short lived sea-level transgressions). Carbonate-Dominated (usually the TST of sequences): An early molluscan skeletal packstone defines a transgressive lag. This is overlain by a peloidal-rich packstone possessing variable amounts of marl, lime mud, and other skeletal fragments (un-abraded brachiopods, bryozoans, and rare echinoderms). This in turn is overlain by a mollusk packstone that likely represents sub-fair-weather wave-base rudist accumulations. The sequence is capped by a molluscan-ooid grainstone to packstone, miliolid packstone to wackestone, quartz-sandy lime mud, algal laminites, and rarely evaporates. This stacking pattern and lack of abundant encrusting allochems suggest a carbonate ramp depositional architecture. Clastic-rich beds may represent tempestite events, or temporary carbonate sedimentation hiatuses (short-lived sea-level regressions).
Detailed petrographic study of well cuttings from deep drilled wells (>1.5 km), integrated with geophysical logs analogous core, and seismic data can be used to produce detailed sedimentologic models and sequence stratigraphic frameworks. Basin-scale depositional trends indicate increased accumulation of carbonate facies in the south, with increased siliciclastic material in the northern regions of the basin. The trend is consistent with trends observed offshore. Namely, the sediments of the Southeast Georgia Embayment to the south are carbonate-dominated while the Baltimore Canyon Trough to the north is clastic-dominated. Despite North Carolina’s tropical setting during the Lower Cretaceous, a significant portion of the studied sediments indicate depositional environments akin to clastic prograding shorefaces (bases are marked by mollusk-rich transgressive lags). Intervals dominated by carbonates most often occur during transgression, or in the distal basinward portions of the basin. Brown, P.M., J.A. Miller, and F.M. Swain, 1972, Structural and stratigraphic framework and spatial distribution of permeability of the Atlantic Coastal Plain, North Carolina to New York: U.S. Geol. Survey Prof. Paper 796, 79 p.
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