THE ENIGMA OF PROGRADING STEEP, HIGH RELIEF CARBONATE PLATFORM MARGINS

Seaward progradation of low-angle platform margins is somewhat like a sand wave, with sediment from the platform cascading down the slope. But steep, high relief margins fronting deep basins can also prograde and as such are somewhat perplexing. How can steep margins prograde, especially when they are cut off from a platform top source of sediment? Characteristics of two prograding ancient examples, one Permian and the other Carboniferous in age, provide a model, which may apply elsewhere.

The Capitan margin rimmed the Delaware Basin of west Texas and southeastern New Mexico during the Late Guadalupian (Capitanian). Information on the morphology and progradational history of this margin comes from subsurface seismic and well data, and also superb outcrops in the nearby Guadalupe Mountains. Although the margin was high-relief (300 to 550 m high) and steep (30 to over 70° dip), progradation around the basin edge extended the margin from 5 to 19 km. Outcrops of steep, high relief Serpukhovian to Moscovian margins in northern Spain serve as important analogs for subsurface hydrocarbon reservoirs in steep-sided isolated platforms of the Pricaspian Basin in western Kazakhstan. Progradation of 4 – 5 km is recorded despite the high-relief (300 to 700 m high) and steep (exceeding 30° dip) nature of these margins.

Several attributes are common to both examples: (1) a highly productive microbial boundstone factory extends over a broad depth range (from the platform break to nearly 300 m depth); (2) stabilization of the margin and upper slope by pervasive marine cementation; and (3) megabreccias and grain flow deposition on the slope of material from the margin and slope itself, with little contribution from the platform top. The broad depth range for microbial boundstone growth increases the potential for this style of carbonate factory to continue production during both lowstands and highstands of sea level and thereby facilitates progradation. Marine cementation provides stability to the steep slopes, but also leads to megabreccia formation after oversteepening and subsequent failure of the slope. What localizes these factors along certain margins and to what extent these factors characterize younger margins has yet to be determined, but microbial activity and marine cementation do occur along modern margins and slopes.