Browsing by Subject "depositional environments"
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Item A Comparison of the Depositional Environment of the San Andres Formation in the Palo Duro Basin to Recent Evaporitic Environments(1984) Chapman, Jenny BurgenThe safe storage of high-level radioactive waste in a geologic repository requires a detailed knowledge of the properties of the host rock and surrounding beds, and the continuity of these properties. One of the serious problems faced in the characterization of the deep formations under consideration is that sufficiently detailed descriptions cannot be obtained from the small number of widely spaced test holes available. The examination of modern analogs to the formation under consideration can provide important insights into geochemical characteristics and their degree of continuity. The first step in identifying a natural analog is to adequately describe the formation of interest and to determine its environment of deposition. The formation under consideration in the Palo Duro Basin, the San Andres Formation, has been described elsewhere (Presley, 1979a & b; 1980a & b, 1981; Presley and Ramondetta, 1981; Ramondetta, 1981; Handford, 1981a & b; Handford and Wiggins, 1981; Bassett and Palmer, 1981; Bassett and Roedder, 1981; Budnik and Smith, 1982; Roedder, 1982). In the Palo Duro Basin, the San Andres Formation is an evaporite sequence containing halite, anhydrite, carbonates, and mudstones. The lithology of the San Andres and the stratigraphic sequence prior to San Andres deposition indicate that the formation was deposited at the end of a long-term shift from fan-delta, marine shelf, and deep-basin environments during the Pennsylvanian to shallow marine, brine pan, and evaporite conditions during Late Permian time. A modern analog environment should be located in a relatively shallow basin that has already been filled by marine sedimentation and is now a broad shelf undergoing long-term oceanic regression and slow subsidence.Item Depositional Systems, Uranium Occurrence and Postulated Ground Water History of the Triassic Dockum Group, Texas Panhandle-Eastern New Mexico(1977) McGowen, J. H.; Granata, G.; Seni, Steven J.Late Triassic (Dockum Group) rocks accumulated in a relict Paleozoic basin defined, in Texas, by the Amarillo Uplift to the north and the Glass Mountains to the south. This basin was reactivated during the Late Paleozoic or Early Mesozoic by tectonic activity that probably was related to the opening of the Gulf of Mexico. The basin subsided, some relict positive elements were uplifted, and sediments began to accumulate in the basin. More than 2,000 feet of terrigenous clastics, derived mostly from older sedimentary rocks, accumulated within the basin. Source areas were in Texas, Oklahoma, and New Mexico; sediment transport was from the south, east, north, and west. The Dockum Group accumulated in a variety of depositional systems including: (1) braided and meandering streams; (2) alluvial fans and fan deltas; (3) distributary-type lacustrine deltas (highly constructive lobate deltas); (4) lacustrine systems including ephemeral and relatively long-lived lakes; and (5) mudflats. Dockum sedimentation was cyclic. The underlying cause of cyclicity was an alternation of humid and arid climate; tectonism most likely was the climatic trigger. During humid climatic conditions, lake level was relatively stable (lake area and depth were maximum). Meandering streams supplied sediment to high-constructive lobate deltas in the central basin area (Texas and New Mexico), whereas braided streams and fan deltas were dominant depositional elements within southern and northern basin areas. Lake area and depth decreased during arid climatic conditions, base level was lowered, valleys were cut into older Dockum deposits, and relatively small fan deltas were constructed along margins of ephemeral lakes. Evaporites, calcretes, silcretes, and soils developed upon floors of ephemeral lakes and on delta platforms. Occurrence of uranium in the Dockum Group has been known for years. An association between depositional facies and uranium occurrence was first documented through research by the Bureau of Economic Geology. Twenty-five distinct depositional facies have been recognized in the Dockum; each of these facies contains uranium. Highest uranium values are in lacustrine facies which developed under arid climatic conditions; however, only a few areas exhibited high values. Channel-lag facies of meanderbelt systems generally exhibit consistently higher uranium values than other depositional facies. Crevasse channel and crevasse splay deposits associated with meandering streams and delta distributaries locally contain carbonized wood, some of which contains uranium. Facies of high-constructive lobate deltas contain uranium; highest values are exhibited by delta front sandstones. Some valley-fill deposits are mineralized; radioactive minerals mostly are within conglomeratic parts of the sequence. Uranium deposits within the Dockum Group are, for the most part, epigenetic and generally occur within sandstone bodies. Four sources of uranium for Dockum mineralization are possible: (1) Triassic volcanics; (2) igneous rocks in Oklahoma; (3) Cretaceous volcanics; and (4) Tertiary volcanics. Although there is a relationship between uranium occurrence and depositional facies, prediction of areas of uranium occurrence is difficult because of a rather complex groundwater history. Groundwater flow was for the most part basinward (down depositional slope) during deposition and shallow burial of the Dockum. Erosion during Jurassic and Early Cretaceous influenced local groundwater flow, which would have been toward erosional lows. Groundwater chemistry was probably affected by marine transgression during the Cretaceous. Following accumulation of Cretaceous sediments, erosion again dominated the area of the Dockum basin; erosion prevailed until sometime during the Pliocene. Pliocene (Ogallala Formation) wet alluvial fan deposits accumulated upon a highly dissected surface underlain in part by Cretaceous rocks, but most of the area was underlain by the Dockum Group. During and subsequent to Pliocene deposition, groundwater flow was to the east in both the Ogallala Formation and the upper part of the Dockum Group. At present, there are two favorable areas for uranium exploration in outcrop: (1) Tule Canyon-Palo Duro Canyon area, and (2) from southern Dickens County southward through Mitchell County. Within the subsurface, a widespread radiometric anomaly occurs at the top of the lowermost progradational sequence; this anomaly is a few hundred to more than 1,000 feet below ground surface. A fourth favorable area for uranium exploration is the uppermost Dockum which has been dissected and is immediately overlain by the volcanic ash-bearing Pliocene Ogallala Formation.Item Sedimentary Facies, Depositional Environments, and Paleosols of the Upper Tertiary Fort Hancock Formation and the Tertiary Quaternary Camp Rice Formation, Hueco Bolson, West Texas(1990) Gustavson, Thomas C.The Hueco Bolson is a segment of the Rio Grande Rift, which formed as a result of late Tertiary Basin and Range deformation. The upper Tertiary Fort Hancock Formation and the upper Tertiary-Quaternary Camp Rice Formation compose the basin fill except in the deepest (western) parts of the bolson. Five lithofacies make up the Fort Hancock Formation: (I) gravel; (II) sand, sandy mud, or sandy silt and gravel; (III) sand, sandy mud, and sandy silt; (IV) clay and sandy clay; and (V) clay, mud, sandy mud, and gypsum. These lithofacies represent the textural gradation from basin margin to basin center of proximal to transitional to distal alluvial fans (lithofacies I through III) to ephemeral lakes (IV) to saline playas (V). In cores from beneath the study area, these same lithofacies are present in a 230-meter-thick (700-ft) upward-fining sequence. The sequence records the lacustrine expansion that occurred over basin-margin alluvial fans as the basin filled. The Fort Hancock Formation is separated from the overlying Camp Rice Formation by a regional unconformity. The unconformity records a period of extensive erosion that marks the integration of the ancestral southern and northern segments of the Rio Grande approximately 2.25 million years ago. Fluvial, lacustrine, and eolian sediments accumulated above the unconformity as the Camp Rice Formation. Five lithofacies also make up the Camp Rice Formation: (1) sand and locally derived gravel, which was deposited by tributaries to the Rio Grande; (2) sand and exotic gravel (derived from north of the study area), which was deposited by a through-flowing stream, the Rio Grande; (3) sand, which was deposited as a dune complex; (4) coarse silt and very fine sand, which was deposited as loess; and (5) clay, sandy clay, and gypsum, which was deposited in ephemeral lakes with central playas.