Quaternary Shelf Edge and Upper Slope Deposition in Low Latitude Mixed Siliciclastic/Carbonate Systems: Lessons from the Gulf of Papua, the Western Texas Shelf, and the Belize Margin

Although siliciclastic/carbonate mixed systems are common in the geological record and interesting because siliciclastic and carbonate components can interact over space and time, modern examples of mixed continental margins remain poorly studied. We present a review dedicated to late Quaternary shelf edge and slope deposition in mixed systems, by integrating recent observations from the Gulf of Papua, the Western Texas Shelf, and the Belize Margin. These regions are characterized by large volume of siliciclastic sand and mud juxtaposed to areas of major neritic carbonate production. These modern analogues are used to develop a quantitative understanding of shelf edge barrier reef edification and sediment transport and accumulation across and along adjacent slopes at different segments of relatively well established sea level cycle. Sharp siliciclastic-to-carbonate switch on the shelf edge and carbonate-to-siliciclastic switch on the upper slopes, correspond to times of initial sea level rise following the Last Glacial Maximum (MIS-2) and the initial onset of sea level fall at the end of last interglacial (MIS-5e), respectively.

Onset of rapid sea level rise during the early deglaciation, when siliciclastics are deposited along newly formed coasts on up dip positions, became opportune time windows for coralgal communities to establish themselves on top of maximum lowstand (MIS-2) coastal siliciclastic deposits, such as beach ridges and lowstand shelf edge deltas. Coralgal communities built 30 to 50 m-high edifices during the first part of last deglaciation in 5 to 6 ky, and then drowned, when sea level rose again after the Younger Dryas, a 1 ky-long cooler interval when sea level remain stable.

On the upper slope, almost pure carbonates, a mixture of mostly bank-derived fine aragonite and magnesian calcite and some pelagic calcite, accumulated during maximum sea level highstand (MIS-5e) when the neritic carbonate production was at its maximum. This carbonate unit is overlain by an almost pure siliciclastic mud unit thinning down the slope. During the late interglacial interval (MIS-5d to 5a) following the maximum highstand (MIS-5e), a rapid ~ 50 m sea fall destabilized thick unlithified siliciclastic muds, stored as today on inner/mid shelf clinothems during the penultimate highstand, through exposure, reworking, transport as suspended material across the narrowed continental shelf, to be ultimately delivered along the shelf margin to the upper slope.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California