Erosion and Deposition Along the Mid-Permian Intracratonic Basin Margin, Guadalupe Mountains, Texas
John C. Harms, Lloyd C. Pray, 1974. "Erosion and Deposition Along the Mid-Permian Intracratonic Basin Margin, Guadalupe Mountains, Texas", Modern and Ancient Geosynclinal Sedimentation, R. H. Dott, Jr., Robert H. Shaver
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Sediments, processes, and the morphologic profile at intracratonic basin margins commonly arc similar to those of the continental shelf, slope, and rise along open oceanic margins. However, the increased potential for sharp density stratification of intracratonic basin waters and for generation of density currents on surrounding epicontinental shelves can markedly influence depositional and erosional processes on the basin margin or floor and can create distinctive sedimentary features that help to differentiate intracratonic and ocean-margin environments of the geologic record. The mid-Permian outcrops of the Guadalupe Mountains provide excellent examples of both depositional and erosional features of an intracratonic basin margin where sharp density stratification and persistent density currents formed by temperature or salinity differences, rather than bysuspended clay, were important sedimentologic factors.
The mid-Permian (Leonardian to early Guadalupian) northwestern margin of the Delaware Basin probably had a normal shelf-slope-rise profile, having several hundred meters of relief, and slopes of a few degrees or less along the basin-margin depositional slope. The exposed 1,000 m of mid-Permian basin-facies strata consist mostly of finely textured dark carbonate rocks, fine-grained sandstones, and siltstones. Carbonate sands and allochthonous carbonate conglomerates and megabreccias derived from the bank or bank margin are locally conspicuous (but minor) interbedded strata. The Leonardian rock units are the contemporaneous bank and basin facies, Victoria Peak dolomites and Bone Springs limestones, respectively. The early Guadalupian rock units are the Cutoff “Shale,” composed mostly of basin-facies limestone, and the overlying Brushy Canyon Formation, composed mostly of detrital sandstone and siltstone. The Cutoff strata lie above and parallel to the basin-sloping unconformity that truncates Leonardian basin-margin deposits. Brushy Canyon strata unconformably onlap both Leonardian and Cutoff strata.
The abruptness and position of the Victorio Peak-to-Bone Springs facies change indicate the sharpness and persistence of a euxinic interface along the lower part of the Leonardian basin-margin slope. Currents were generally weak or absent near the interface, but erosion surfaces, some overlain by sheets and channel fills of bank-derived carbonate sands, indicate episodes of higher competence of bottom currents.
Intra- and interformational erosion features are more prevalent in Guadalupian strata. Two major erosional phases created unconformities at both the base of the Cutoff and the Brushy Canyon rock units. The unconformities at the basin margin slope 5° to 10° basinward. The lower one truncates about 250 m of Leonardian basin-margin strata, and its carving required appreciable retreat and steepening of the basin-margin depositional slope. The upper unconformity forms the onlap surface for more than 300 m of Brushy Canyon deposits. Several steep-sided, narrow channels as much as 40 m deep incise the sloping unconformity surfaces. Erosion concomitant with sedimentation of basin facies persisted throughout early Guadalupian deposition, and basin- trending channels are especially well displayed in the Brushy Canyon. Brushy Canyon intraformational channel dimensions are substantial, as depths may exceed 25 m, widths 1 km, and lengths many kilometers. Brushy Canyon channels are filled in part by beds of sandstone containing upper flow-regime features that conform to the flatter channel floors and that abut adjacent channel walls. Finely laminated siltstone beds mantle channel floors, walls, and interchannel areas and form the bulk of the Brushy Canyon deposits.
The erosive agents that cut both channels and unconformities left clean, smooth contacts but little evidence of their nature. We believe that density currents were the major erosive agent and that all erosion occurred in a relatively deep submarine environment. Evidence for submarine origin includes the basin-facies character of all deposits overlying erosional surfaces, the similarity of small and large scale erosion surfaces, the similarity of the Brushy Canyon erosional features to those of later Guadalupian deposits of established deep-basin origin, and the absence of recognized features of subaerial, vadose, or shallow-marine environments. If sea-level changes were involved, the sea may have been deeper rather than shallower during the carv ing of the major unconformities.
The erosional and depositional features of the mid-Permian basin margin are compatible with a basin having sharp density stratification and with frequent spilling of shelf-generated cold or saline density currents down the shelf margin. Denser, bottom-hugging currents carved the channels and probably the unconformities and deposited the coarser grained carbonate and detrital sands. Less dense currents moved partly down the slope and then spread far out over the basin as interflows, creating a rain of finer grained sediment on the deeper basin floor. Density currents may have been frequent, of long duration, and not limited to master channels, thus minimizing proximal-to-distal and fan apex-to-interfan contrasts. Episodic phenomena expectable on any marginal slope, such as debris flows that carried very coarse clasts several kilometers into the basin, or slumps, or perhaps deep wave action, contributed to the sedimentary features. The mid-Permian sedimentary prism of the intracratonic Delaware Basin provides some marked contrasts as well as similarities to sedimentary prisms fronting open ocean basins. In its overall features, it is significant that here is another example from the geologic record for which there appears to he no reasonably close modern analog.