Title: EROSION AND DEPOSITION ALONG THE MID-PERMIAN INTRACRATONIC BASIN MARGIN, GUADALUPE MOUNTAINS, TEXAS
Abstract: Sediments processes and the morphologic profile at intracratonic basin margins commonly are 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 su rounding epicontinental shelves can markedly influence depositional and erosional processes on the basin margm or floor and can create distinctive sedimentary features that belp 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 by suspended 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 tbe 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 cbange 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 tbe interface hut erosion surfaces some overlain by sheets and channel fills of bank derived carbonate sands indicate episodes of higher competence of bottom currents Intra and intedormational erosion featnres arc more prevalent in Guadalnpian strata Two majo erosional phases created unconformities at both the base of tbe Cutoff and the Brushy Canyon rock units The uncon formities at the basin matgin slope 50 to 100 basinward The lowcr one truncates about 250 m of Leonardian basin margin strata and its carving required appreciable retreat and steepening of the basin margin deposi tional slope The upper unconformity forms the onlap surface for more than 300 m of Brusby Canyon deposits Several steep sided narrow channels as much as 4 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 inclndes the basin facies cbaracter of all deposits overlying crosional surfaces the similarity of small am large scale erosion surfaces the similarity of the Brushy Canyon erosional features to those of later Guadalupian deposits of established deep basin origiu and the absence of recognized features of subaerial vadose or shallow marine environ ments 1 f sea Icvel changes were involved the sea may bave been deeper rather than shallower during the carv ing of thc 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 tbe 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 pbenomena expectable o any marginal slope sucb 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 Tbe 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 tbat here is another example from the geologic record for which there appears to be no reasonably close modern analog
Publication Year: 1974
Publication Date: 1974-01-01
Language: en
Type: book-chapter
Indexed In: ['crossref']
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Cited By Count: 2
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