科研之友
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摘要
Steep, debris-rich, progradational carbonate slope systems have been well-documented around the world in a variety of settings and time periods. While the association of such slopes with deep microbial boundstone upper slope factories, the planar nature of their clinoform profiles, and their toe-of-slope trajectory patterns have been discussed thoroughly, the internal architecture and genesis of clinothems and the mechanics behind progradation remain poorly understood. Capitan Formation (Seven Rivers and Yates Formation equivalent) exposures in the southern Guadalupe Mountains of west Texas allow for examination of steep, prograding Upper Permian (Guadalupian) carbonate slope strata that are coeval with deep, skeletal-microbial boundstone reefs positioned on the upper slope (100-200 m water depth). Measured section data tied to interpreted photomosaics and hand samples were collected within a 7 square-kilometer study area in Pine Canyon and the Frijole wall to document the deposits and architecture of the Capitan slopes. A subset of a regional airborne Lidar dataset was utilized for mapping declivities and orientations of clinoform surfaces.
The outcrops reveal that Capitan middle to lower foreslopes are dominated by lenticular breccia deposits originating from the brittle failure of upper slope boundstone factories, with lesser intercalations of platform-derived carbonate grain-dominated deposits and bypassed siliciclastics. The observed lenticular carbonate breccia deposits stack hierarchically and exhibit lateral and upslope offsetting (compensational and backfilling) architecture in response to the slope topography of previously deposited debris. Over time, this process constructed lower to middle slope conical sediment bodies that coalesced laterally to form strike-extensive, debris-dominated aprons. Once these debris aprons filled the slope profile up to the level of in situ microbial boundstone production, substrate became available for the upper slope reefs to prograde. The advance of the boundstones overtop their own debris completed the development of a shelf-to-basin debris-dominated clinothem, and the process repeated resulting in boundstone margin and foreslope progradation.
These Capitan foreslope architectures indicate that sustained, small-scale gravitational failure of the upper slope boundstones was the dominant resedimentation process that contributed to slope development. Single collapse events that produced individual breccia beds occurred at a much greater frequency than relative sea level fluctuations recorded on the platform-top, and variability in breccia matrix sediment suggests that boundstones were shedding material downslope throughout all stages of the accommodation cycle. We propose that boundstone accretion, local instability and failure, and resulting slope breccia deposition were somewhat independent of allocyclic, high-frequency accommodation changes, and that an autogenic control associated with deep oligophotic microbial margins is dominant during the development and progradation of steep, debris-dominated slope systems.
- 出版日期2018-8
