The Jurassic J-2 surface is one of the more extensive unconformities of the Western Interior (U.S.A.). Stratigraphic relationships show that this is an angular unconformity, which developed first in the north and progressively moved south. An examination of features associated with the J-2 surface reveals detrital and diagenetic chert grains, ventifacts, polygonal fracture patterns, oscillation ripples, and paleotopographic relief. Some of these features indicate that the J-2 unconformity was a hot, dry, deflationary surface, and that the level of deflation on this surface was controlled, at times, by the height of the water table. The ventifacts are a lag deposit of the overlying Gypsum Spring Formation. Three possible variables involved in the formation of the J-2 surface are fluctuations in water table, disruption of sediment supply, and tectonic activity. For the Early-Middle Jurassic, these variables can be associated with increased rifting rates in the Atlantic Ocean. The increase in rifting rate raised eustatic sea level, which resulted in a larger area for deposition of marine sediments and a rise in continental water table. A higher water table would have trapped sediment that was previously available for sand sea (erg) formation. Furthermore, a higher sea level might have changed sediment supply and transportation routes. As the increase in rifting continued, tectonic movement and tilting of the North American plate occurred. This tilting caused the uplift and erosion of some deposits, and influenced the sites available for sediment preservation. / text

The Hickory Sandstone Member of the Riley Formation is dominantly quartz sandstone up to 167 m thick which crops out in the Llano Uplift region of central Texas and dips away in all directions. It lies unconformably upon the irregular surface of the Precambrian Texas craton. The association of isopach thicks and thins over cratonic lows and highs demonstrates topographic control of Hickory deposition. Regional subsurface studies delineate the extent of the overlying Cap Mountain Limestone. Beyond the limits of the Cap Mountain, the Hickory grades into the Lion Mountain Sandstone laterally and vertically so that correlations are difficult. The six lithofacies of the Hickory Sandstone were deposited as nonbarred tidally-influenced or estuarine-related equivalents to deposits of Holocene environments. Outer estuarine tidal channel-shoal deposits display abundant channel fills of large-scale foresets, parallel bedded sandstone, and minor siltstone. Trilobite trackways (Cruziana) and resting traces (Rusophycus) occur in these deposits, associated with U-shape burrows (Diplocraterion and Corophioides). Deposits of open coast sandy tidal flats display upward-fining character, medium-to large-scale festoon crossbedding, abundant small-scale ripple bedforms of all types, and mudcracks. These deposits include the U-shape burrows, Corophiodes, and the trackway, Climactichnites. Deposits of inner estuarine tidal channels and tidal flats display upward-fining character, wavy-lenticular bedding, bimodal paleocurrent patterns, and the resting trace, Pelecypodichnus. All of these deposits prograded as a unit until sea level rise shut off sediment supply. Progradation of tidal channel and shoal sediments was renewed. These deposits are festoon crossbedded hematitic sandstone with wavy-lenticular bedding and abundant fossil debris. Storm energy funneled through tidal channels deposited crossbedded sandstone onto the nearshore inlet-influenced shelf. Final Hickory deposits and initial Cap Mountain deposits were storm-dominated, burrowed and laminated calcitic shelf sands. / text

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.

Not available / text

The Earth’s atmosphere has undergone several stages of progressive oxygenation throughout its history which has had profound effects on the behaviour and availability of metals on the Earth’s surface, and the biosphere. A broad range of geological and geochemical evidence has been used to reconstruct the stages of the atmosphere’s oxygenation. However, there is a large gap in data between ~1.8 and 0.8 Ga, leading to the assumption that there were only minor changes in the oxygen content of the atmosphere during that time. Most geological and geochemical data from this stage, known as the ‘boring billion’, is derived from rocks deposited in deep-marine environments, which had little interaction with the atmosphere. During the boring billion the Earth’s crust was undergoing a period of hitherto unparalleled continental assembly, forming the supercontinent Rodinia. Crustal differentiation following the amalgamation of Rodinia caused the concentration of metals in the upper crust. Such a large continental mass also allowed intracontinental basins to form resulting in the deposition of terrestrial sedimentary successions, which have much greater interaction with the atmosphere than those deposited in deep environments. Thus terrestrial rocks of boring billion age are more likely to contain geochemical evidence of the atmospheric oxygen content than their deep marine counterparts. One such succession is the Torridonian Supergroup of NW Scotland. Analysis of the facies and metal deposits from varied depositional environments within the Torridonian and other contemporaneous terrestrial successions as part of this study has yielded evidence suggesting the atmosphere and surface environment was considerably more oxygenated, and that metal availability in the surface environment was much greater during the boring billion than previously thought. Furthermore, evidence in this study suggests the biosphere had adapted to inhabit an oxygen-rich terrestrial environment, evolved to utilise increasing availability of trace metals, and had a critical role in the concentration of metals in the Earth’s surface during the mid-Proterozoic.

Detailed mapping (1:6,000) of Lower Ordovician through Lower Pennsylvanian strata, exposed in the vicinity of the Sunshine Springs thrust fault, shows that the thrust ramps up-section twice in a direction parallel or subparallel with the thrusting, and that the geometry of folds can be attributed to their proximity to the two closely spaced ramps. The lower ramp is a frontal ramp which originated as a forelimb thrust through the overturned limb of a tight anticline-syncline fold couplet. The upper ramp cuts up-section through a thin, upper Paleozoic flysch sequence where the Sunshine Springs thrust becomes imbricated. Directly above the lower ramp, in the upper plate, is a broad symmetrical anticline which has a geometry similar to a fault-bend fold. Forward of the lower ramp is a large wavelength, flat-bottomed syncline, and behind the lower ramp is a series of tight to isoclinal overturned folds. As a result of fault-bend folding and continued shortening of the ramp region, upper plate folds characteristically have a larger amplitude than folds of the lower plate. Just forward of the lower ramp in the footwall is the tightly folded and truncated syncline of the syncline-anticline fold couplet. The rest of the lower plate section is only mildly deformed. A composite, down-structure cross section drawn parallel with the direction of thrusting shows that the Peña Colorada synclinorium has been transported along the Sunshine Springs thrust approximately 3.8 km. Shortening, as deduced from folding in this study alone, is 20 percent, and when the shortening by the thrust is also considered, the total amount of shortening equals 52 percent. A major left-lateral strike-slip system, trending WNW, approximately parallel with the thrusting direction, offsets the Sunshine Springs thrust fault. Strike-slip and dip-slip displacements can be calculated from a displaced fold axis of the lower plate syncline, and are 335 m and 90 m, respectively. In the vicinity of this strike-slip system, the axial traces of folds change from a dominantly southwesterly trend to a more southerly trend. The regional extent of the fault system within the Marathon Basin, and its correspondence with the change in major fold axes orientations suggests that the fault zone is a regional tear which formed in response to the impingement of the Marathon thrust front against the Diablo Platform during the Pennsylvanian Period. / text

The Rim Rock country, on the western edge of a vast Trans-Pecos Texas lava field, received thousands of feet of volcanic material during the Tertiary Period. Although pyroclastic rocks predominate, several flow rocks form the most distinctive stratigraphic markers. Dikes, sills, and laccoliths were emplaced following the cessation of extrusive igneous activity. The Vieja Group, comprising the oldest volcanic strata, has been tentatively assigned to the Chadronian Stage (Lower Oligocene). Late Tertiary block-faulting created an intermontane area which subsequently received thick bolson deposits. Recent climatic fluctuations have resulted in the development of gravel-capped terraces along the Rio Grande. / text

The Birrimian System forms most of the exposed Precambrian shield of West Africa. About half of Ghana is underlain by Birrimian metamorphic and granitic rocks. In the Dunkwa S. E. area (Field Sheet 48) of south-central Ghana, interlayered sedimentary and volcanic rocks were metamorphosed to greenschist and amphibolite facies. These sheared and tightly folded metamorphic rocks have incipient retrograde metamorphism of biotite and garnet to chlorite or hornblende to biotite. Two large intrusive bodies in the metamorphic terrain may be partly the result of granitization: (1) the Pra River complex, an albitized laccolith or sill, consisting of quartz monzonite, quartz diorite, and granodiorite; (2) the Twifu Praso complex of albite granite and granodiorite gneiss. All of the above rocks were folded along northeast-trending fold axes. Then a third granitic body, the Wuwu River albitized quartz monzonite, intruded and locally re-folded the older rocks. / text

Paleozoic, Mesozoic and Cenozoic rocks totaling about 15,000 feet in thickness are exposed in the Cowboy Pass area in the Confusion Range. The Paleozoic sedimentary rocks are carbonates, with small amounts of sandstone and shale, and the Mesozoic sedimentary rocks are shallow water limestone and shale. Cenozoic deposits are alluvium, lacustrine beds and small amounts of volcanic material. The trend of major folds and faults changes sharply from northerly to northeasterly at Cowboy Pass, and this change is accompanied by minor faulting and folding. Major structures are the result of post Lower Triassic-pre Cenozoic orogeny. During Cenozoic time, high angle normal faults developed, accompanied by local volcanic activity. / text

Detailed mapping of Carboniferous rocks westward from areas mapped by Bogardus and Oden shows that rock units within the Marble Falls formation are traceable across "Cavern Ridge" a "barrier" invented by Plummer and referred to by others. The Ives breccia, Chappel limestone, Barnett formation, and Lower member of the Marble Falls formation formed as a transgressive depositional sequence. A thin zone of bypassing separates the Barnett and Marble Falls, but a hiatus between them in the Hall area cannot be demonstrated. A small-scale disconformity exists between the Lower and Middle members of the Marble Falls. Local faulting during deposition of the Marble Falls produced the Gibbons conglomerate, influenced the accumulation of Middle Marble Falls shale, and elevated the "Hall Uplift." Some lithosomes in the Upper member of the Marble Falls pinch out near the "Hall Uplift." Sandstone and mudstone of the Strawn formation abut against the carbonate mass of the Marble Falls to the north, but whether a period of erosion intervened between the deposition of the two is not known. Analysis of seven species of corals, twenty-three of brachiopods, two of pelecypods, five of gastropods, two of cephalopods, two of trilobites, and fourteen of conodonts suggests that the Chappel, Barnett, and Marble Falls faunal assemblages most closely resemble those of the Chouteau formation, Caney formation, and Morrow group respectively. In the Hall area no part of the Marble Falls should be correlated with the Atoka series. / text