Lower Cretaceous strata in central Texas are divisible into genetically related rock bodies. The Lower Trinity Group contains a lower terrigenous near shore deposit (Hosston Formation) which interfingers with an overlying shallow water, offshore limestone-dolomite sequence (Sligo limestone); this Group may contain a barrier reef section downdip. The Middle Trinity Group, a sequence of grey to black shale and limestone, does not intercalate with either the underlying or overlying rock bodies. Updip, near the Texas craton hingeline, it is separated from the overlying rocks by a disconformity. Downdip the Stuart City reef, a rudistid limestone, exists continuously between the top of the Middle Trinity Group and the upper part of the Georgetown formation of the Washita Division. Back reef deposits are separable into Upper Trinity Group, Fredericksburg Group and part of the Washita Division. The Upper Trinity Group consists of the basal Hensel sand (mostly updip outside the area of study) which intercalates going downdip with the usually dolomitic Glen Rose limestone. The middle part of the lower Glen Rose contains a reefal limestone section which is restricted to the area near and over the San Marcos arch and near the Stuart City reef. The upper Glen Rose is a sequence of dominantly evenly-bedded, dolomitic, foraminiferal limestone. The Fredericksburg Group includes the East Texas basin lagoon deposits of the Walnut and Comanche Peak formations and the rudistid limestone deposits of the lower Edwards. The lower Edwards intercalates with the lower units of the Walnut over the San Marcos arch and with successively higher units toward the East Texas basin; lower Edwards and Comanche Peak produce a similar pattern. The basal unit of the Washita Division is a widespread black shale (Kiamichi) and shaley limestone (middle (Kiamichi) Edwards) which is absent by onlap around the Belton high (proposed new term). The dolomitic, rudist limestone of the upper Edwards interfingers with the lower beds in the Georgetown limestone north of the San Marcos arch. The remainder of the Georgetown and the overlying Del Rio clay are laterally continuous throughout the area of study. The Buda limestone pinches out near the edge of the Texas craton and thickens going southeast and downdip.

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The Upper Cretaceous (Campanian) Cedar District Formation of the Gulf Islands and adjacent areas is composed of shale and sandstone, which are present as thick sequences of shale, which may or may not be fossiliferous, and as alternating rhythmic sandstone-shale sequences of the flysch-type. Presence of graded bedding, ripple and convolute laminations, and sole marks in the latter suggest a turbidity current origin. The internal structures of the individual turbidite units correspond largely to the C-E divisions of Bouma (1962) and other authors, and indicate that their deposition took place largely within the lower flow regime. Convolute lamination in the sandstones was formed by oversteepening and deformation of pre-existing ripple lamination and by the deformation of pre-existing plane-parallel lamination by the drag of the overpassing currents. Flute and groove casts and frondescent marks were only found in beds thicker than a foot and a half. Calcareous concretions, most abundant in the shales and occasionally phosphatic, are crossed by organic borings and burrows which are filled with sediments of the surrounding beds. Host rocks of the calcareous concretions tend to thicken around them. The concretions show deformation when present in beds involved in soft-sediment deformation. All these observations suggest their formation in the early stage of diagenesis, probably shortly after, burial. Sandstones of the Cedar District Formation show a gradation from arenites that lack matrix and have a cal-cite cement, to wackes rich in fine-grained matrix. The majority of the wackes and the arenites are feldspathic and arkosic, using the classification of Gilbert (1954). Their composition indicates that the major source was acidic to intermediate igneous and/or low to medium grade metamorphic rock , sedimentary and volcanic rocks were a secondary source. The major source area was possibly a region of high relief that had undergone rapid uplift and erosion, and experienced mainly mechanical weathering. Paleocurrents and lithologic lateral variation indicate that the major source area for the coarse elastics was situated to the east and southeast of the study area. The pre-Jurassic low grade metamorphic rocks of the Cascade Mountains to the east, and the pre-Carboniferous- crystalline rocks of the San Juan Islands to the southeast served as possible source areas for the coarse elastics. Deposition of shaley, fossilif erous parts, of the formation in the southeastern part, of the study area, took place in littoral to upper neritic depths. Turbidite (flysch-type) sequences were deposited in deeper water, below the wave base. The unfossiliferous shale of the central and northern parts of the study area was deposited either at about the same depths as the turbidites, or in deeper water, since thin, delicate, horizontal and-cross laminations are preserved in these rocks. Paleontologic evidence suggests that deposition took place in a somewhat restricted basin having a narrow connection with the open ocean to the west. Paleontologic and mineralogic data suggest that the bottom conditions of the central and northern parts of the basin of deposition were stagnant and reducing. Facies relationships suggest that the basin of deposition had its longest dimension trending SE-NW. Its eastern, southeastern, and southern boundaries were situated between the mainland of British Columbia-Washington and the Gulf-San Juan Islands. Its northern and northwestern boundaries were possibly near the city of Nanaimo and Gabriola Island. To the west, it was connected at least partially to the open ocean. In the southeastern part of the study area, alternation of thick, fossiliferous shale sequences, and sequences which are predominantly turbidites suggests fluctuations in the depth of the basin floor, either due to changes in sea level or to tectonic movements. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

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Graduation date: 1991

The major purpose of this study was to examine in detail the anomalous transgressive "boundary" sandstones which generally are present at formation boundaries throughout the Parras and La Popa basins. These extensive sheet-like sandstone bodies are massive, resistant units up to 20 m thick and up to 25 km wide that form the cap of mountains and hogbacks. These are anomalous units because transgressive deposits thicker than 5-10 m are rare in ancient deposits although they are well-documented for some Holocene deposits. For example there are no known thick transgressive deposits in the well-studied cyclic transgressive-regressive sequences in the Cretaceous rocks of the Rocky Mountains. McBride and others (1975) previously interpreted these boundary sandstone bodies to have been laid down as thick transgressive or regressive blanket deposits as the shoreline moved back and forth across the basins in response to variable basin subsidence and sediment input. In order to clarify this interpretation, we selected deposits formed during the latest part of the first deltaic progradation represented by the Cerro Huerta Formation, and at the beginning of the first marine transgression represented by the Cañon del Tule Formation for detailed study. / text

The Lower Cretaceous tectonosequence constitutes a major chronostratigraphic interval in the Recôncavo Basin, Northeast Brazil. It encompasses a time span of ca. of 24 Ma and forms a continental sedimentary package that exceeds 5 km in thickness within its depocenters. The tectonosequence is bounded by two basinwide unconformities, the rift onset unconformity at its base and the breakup unconformity at its top. The basal deposits of the tectonosequence characterize transitional rift conditions. The bulk of the succession constitutes the record of the rift phase in the basin. Biostratigraphy in the Recôncavo Basin is based on nonmarine ostracods. The tectonosequence encompasses ostracod zones NRT 002 through NRT 009. Tectonic, sedimentologic, and paleoecologic characteristics permit the recognition of six tectono-depositional intervals within the tectonosequence. Tectono-depositional intervals cover a time span between 2 Ma and 7.5 Ma, and their boundaries are distinct marker beds. Lithologic, paleontologic, and geochemical characteristics of the marker beds suggest low sediment input at the time of their deposition. Tectonic and/or paleoclimatic changes may account for phases of marker bed creation. Within tectono-depositional inteivals II, IV, and V sedimentary cyclicity occurs. Sedimentary cycles spanning tens to hundreds of thousands years are termed tectono-depositional episodes. Such an episode is comparable in scale with a parasequence, and is also bounded by marker beds reflecting flooding events. Depositional systems within the tectonosequence show a change from lacustrine-dominated sedimentation at the base to fluvial-dominated deposition at the top. Reservoir potential increases toward the top of the tectonosequence, source rock and sealing conditions, however, follow the opposite path. The bulk of known hydrocarbon accumulations in the Recôncavo Basin occurs at the prerift/rift interface. Accumulation rate estimates show increasing values toward the top of the tectonosequence. This may be related to a better integration of the drainage system that fed into the basin and removal of possible structural barriers located along the basin's margins. Analysis of the sedimentary record of the tectonosequence suggests, besides tectonism, paleoclimate was an important mechanism controlling facies distribution and evolution. / text