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Abstract

Basin-margin, Late Guadalupian carbonate strata contain units with alternating baselap patterns. Downlap occurs at the base of steeply dipping boulder-conglomerate lower slope strata; onlap occurs at the base of gently dipping wackestone-rich toe-of-slope strata. The downlap and onlap stratal patterns reflect differences in gravity-flow deposition in response to different sediment type supplied to the basin margin. The lower slope was too steep for deposition of peloidal carbonate muds transported by low-density turbidity currents, so gravity flows bypassed the slope and deposited carbonate mud-rich beds at the toe-of-slope, where they onlapped lower slope boulder conglomerates. Matrix-poor boulder conglomerates terminate by downlap in talus cones because the rock falls and low-matrix debris flows responsible for their transport reached a slope too gentle for continued transport. In contrast, steeply dipping, matrix-rich boulder-bearing debris-flow deposits interfinger downslope into finer grained debris-flow and turbidity current deposits concordant with underlying strata.

The sediment type delivered to the basin margin varied systematically during the Late Guadalupian due to sea-level fluctuations. Relative sea-level stand was interpreted from correlation to the relative sea-level record of contemporaneous shelf strata and from analogy to the Bahamian Quaternary carbonate sediment history. Siliciclastic silts were deposited in the basin during lowstands as onlapping strata. Downlapping silt-matrix boulder conglomerates (units 1 and 7) were deposited during the early transgression, when fringing reefs supplied boundstone boulders and siliciclastic silt could be transported across the emergent shelf. Downlapping matrix-poor boulder conglomerates (unit 2) were deposited during the late transgression, when fringing reefs were still growing and the shelf was flooded enough to stop the basinward transport of quartz silt but was not flooded enough to pro-duce significant quantities of carbonate mud. Onlapping toe-of-slope wackestones (unit 3) were deposited during early highstand, when the flooded shelf was producing carbonate mud and the reefs either had not caught up with sea level rise or had stepped back onto the shelf. Boulder conglomerates with lime-mud matrixes (units 4, 6, and 7) formed after reefs had caught up with sea-level rise or had prograded to the shelf edge (late highstand).

The stratal patterns in these Permian, basin-margin carbonates are different from those commonly interpreted in generalizations of siliciclastic sequences (e.g., Vail et al., 1984). The Late Guadalupian stratal patterns are caused by the change in carbonate sediment type and quantity with change of relative sea level, rather than by proximity to shoreline with changing relative sea level. This study demonstrates the importance of understanding the relationship between sea level, sediment supply, and depositional mechanisms before using stratal patterns to interpret relative sea levels in carbonate basin margins.

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