Petrology and enviornments of deposition of the percha formation, upper devonian, southwestern New Mexico

dc.contributor.committeeMemberChafetz, Henry S.
dc.contributor.committeeMemberMaddocks, Rosalie Frances
dc.contributor.committeeMemberMote, Victor L.
dc.contributor.committeeMemberWalker, Robert H.
dc.creatorKocurek, Gary
dc.description.abstractThe Upper Devonian Percha Formation of the southwestern New Mexico is a transgressive terrigenous-carbonate deposit that accumulated in a complex of depositional environments that grade laterally and vertically from tidal flat to shallow lagoon to carbonate platform. The Percha unconformably overlies Silurian strata. The basal 0 to 32 m of strata consist of a variable sequence of distinct lithofacies. In the northern part of the study area, these deposits show a vertical gradation from fissile shale, showing an irregular alternation of silty lenticular and clayey laminae, to more regularly laminated interbedded siltstone and mudstone. This sequence of strata was deposited on a transgressive tidal flat. In the southern part of the study area, this tidal sequence is repeated, but the shales and mudstones are calcareous and overlie quartz sandstone. Carbonate accumulation may have been initiated by diminished terrigenous influx, resulting in tidal flat progradation after the initial deposition of a transgressive beach sand. Rhythmically laminated carbonaceous limestone beds occur at both the northern and southern study areas and were deposited in lagoons isolated from terrigenous influx. Tidal deposits in the northern area are overlain by 44 to 74 m of carbonaceous finely laminated, fissile, black shale that grades upward into silty lenticularly laminated, bioturbated, green shale. This shale sequence was deposited in a transgressive, shallow, euxinic lagoon barred from open circulation by the offshore carbonate buildup. At the southern area, where this lagoonal sequence is 30 m thick, the shale is highly calcareous, fossiliferous, and shows a greater abundance of current-deposited silty laminae. Lagoonal conditions were less restricted at the southern area because carbonate accumulation, initiated during tidal flat deposition, maintained the lagoon bottom at a shallow depth, where it was stirred by wind-generated waves and currents. In the northern area, the upper 4 to 14 m of the shale sequence contains horizontal rows of discrete carbonate lenses a few centimeters in length. In the southern area, this zone is 18 m thick, and carbonate units occur as discontinuous layers several meters long. These carbonate lenses and layers formed in shallow channels filled by fine skeletal debris. This fine carbonate sediment was derived from the offshore carbonate buildup and transported into the outer edge of the lagoon. Channel-fills at the northern area have broken up into discrete lenses by loading. Shale with carbonate lenses and layers grades upward into 8 to 28 m of strata composed of crinoidal ripples separated by a few centimeters of carbonate mud. This zone of flaser bedding is cut by numerous, shallow, migrating channels. This sequence of strata was deposited on the inner carbonate platform. Flaser bedded carbonates grade upward into 1 to 38 m of crinoidal and bryozoan skeletal sand beds separated by a few centimeters of carbonate mud. Individual skeletal sand beds are up to 1 m thick but are highly variable in thickness, and they show wavy, flaser, and lenticular bedding, cross-stratification, and parallel laminations. These strata were deposited in the high energy outer carbonate platform where a broad crinoidal and bryozoan garden formed a wave-baffling bank maintained at or near wave base. The Percha carbonate buildup was unlike most modern platform margin buildups because it lacked a major bound-stone wave barrier. As a result, upwelling currents at the platform edge were only gradually diminished over the broad platform, and outer platform conditions extended over a broad area of the craton. The lithology and sedimentary structures of the Percha are transitional into the overlying Mississippian strata, and no break in the depositional package is present. This contact has been described by previous workers as unconformable, based on the fauna. The absence of a complete "time record" is the result of submarine erosion and the nature of sedimentation.
dc.description.departmentEarth and Atmospheric Sciences, Department of
dc.format.digitalOriginreformatted digital
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dc.titlePetrology and enviornments of deposition of the percha formation, upper devonian, southwestern New Mexico
dc.type.genreThesis of Natural Sciences and Mathematics, Department of of Houston of Science


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