Browsing by Subject "Geology, Stratigraphic--Paleozoic"
Now showing 1 - 4 of 4
Results Per Page
Sort Options
Item Paleontology and sedimentology of the Haymond boulder beds (Martin Ranch), Marathon Basin, Trans-Pecos Texas(1987-08) Witebsky, Susan; Muehlberger, William R.A boulder bed unit in the upper Haymond Formation (Pennsylvanian), generally believed to be olistostromes, is exposed in the eastern Marathon Basin, west Texas. Two localities of this unit (Housetop Mountain and Clark Butte) contain clasts derived from several formations found within the basin, as well as exotic Devonian metamorphic and volcanic rocks. This report describes a third previously unstudied site (Martin Ranch locality) that contains clasts of exotic Middle Cambrian shelf limestones. These limestones provide a key to the Early Paleozoic history of the Marathon region. The boulder beds lie in the upper part of the Haymond Formation. At the Martin Ranch locality they form a zone that is traceable for 6.6 km along strike and is up to 230 m thick. These boulder beds contain interbedded units of massive, unstratified, pebble- to boulder-bearing mudstone, thickly bedded, massive sandstone, lenses of pebbly sandstone, and deformed flysch beds. About 80 percent of the clasts found in the boulder beds at Martin Ranch are chert derived from several basin formations. Unique displaced slabs of bedded chert pebble conglomerate comprise about 10 percent of the clasts. Theses conglomerates were probably derived from upper fan-channel deposits within the lower Haymond Formation. Pennsylvanian limestone clasts redeposited from the basin facies of the Dimple Formation and clasts of exotic, late Middle Cambrian limestones each comprise about 5 percent of the clasts. These Cambrian limestones, older than any formation in the Marathon Basin, contain a fauna characteristic of the seaward edge of the cratonic carbonate shelf. The presence of the Cambrian clasts constrains the location of the North American shelf edge during the Cambrian, placing it at least 120 km southeast of the present day Marathon Basin. Both the Martin Ranch and Housetop Mountain boulder beds are composed mainly of clast-bearing, matrix-supported mudstone which have pebbly sandstone, massive sandstone, and flysch beds interstratified with the mudstone and represent periodic deposition of debris flows, slumps, slides, and turbidites interspersed with normal basin deposition of flysch facies rocks. However, different clast types are found at the two localities. The Martin Ranch locality has clasts of Cambrian limestone and chert pebble conglomerate, the latter up to 90 m in length, that are absent at the other localities. Exotic Pennsylvanian limestone clasts and exotic Devonian metamorphic and volcanic rocks, common at Housetop Mountain, are rare or missing at Martin Ranch. The Clark Butte locality is unique because it lacks the mudstone which dominates the other two localities. Instead, the matrix is composed of a pebbly sandstone and conglomerate associated with thick sandstone beds. The boulder beds at this locality may represent upper fan channels and channel-lag deposits. The turbidites and olistostromes resulted from recycling of the southern edge of the tectonic basin as the advancing Ouachita thrusts uplifted the pre Haymond strata. Most of the clasts were from older basin formations exposed by these faults; however one of these thrusts also uplifted slivers of exotic Middle Cambrian limestone. Earthquakes probably triggered slumps and rock falls off the fault scarps. As the boulders travelled downslope plowing through the slope sediments, they accumulated more material. This combination of slide debris and slope mud turned the slumps and slides into debris flows. Between episodes of debris flows and turbidity currents, normal basin deposition of thinly bedded turbiditic sandstone and pelagic shale occurred.Item Paleozoic geology of the Round Mountain area, Blanco quadrangle, Texas(1929) Hancock, William Tarrant; Not availableItem Paleozoic geology of the Shovel Mountain area, Blanco quadrangle, Texas(1929) Winkler, Hans; Not availableItem The structural evolution of the Sunshine Springs thrust area, Marathon Basin, Texas(1984-08) Kraft, Jennifer Lucille; Muehlberger, William R.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.