This article utilizes over 3000 biostratigraphic reports of fusulinid taxa in the Midland Basin to produce a series of chronostratigraphic surfaces that show shelf-to-basin profiles from the end of the Atokan to the end of the Leonardian. The position of the shelf-edge break along the Eastern Shelf was geometrically reconstructed for each of the chronostratigraphic surfaces. Comparing the location of the shelf edges produced in this study to published examples resulted in significant disagreement for some time intervals, especially the Wolfcampian. These discrepancies are inferred to be predominantly the result of lithostratigraphic-based vs. biostratigraphic-based data. Assessing shelf-edge trajectory through the Pennsylvanian into the early Permian indicates that (1) tectonic and eustatic increases in shelf accommodation resulted in retrogradation of the shelf edge during the Pennsylvanian, and (2) early Permian progradation was the likely result of cessation in tectonic subsidence that allowed bypass of the shelf and passive filling of the basin center. When placed into the context of the Ancestral Rocky Mountains, subsidence analysis of the Midland Basin agrees with tectonic models that portray a synchronous start rather than an east-to-west migration of peak subsidence. Additionally, a relatively synchronous apex of tectonic subsidence occurred in the Middle to Late Pennsylvanian.

The Ancestral Rocky Mountains (ARM) represent an intraplate deformational event that resulted in a series of Precambrian-cored basement uplifts with adjacent basins that accumulated Pennsylvanian to early Permian strata. Tectonic models for the event are debated largely because of the lack of robust age control in the basin fill. In New Mexico, the ARM event resulted in a series of basins with some of the best biostratigraphic records across the orogenic province. This article utilizes published biostratigraphic and lithostratigraphic data to (1) reconcile the onset of subsidence by first accumulation of Pennsylvanian strata, (2) establish a period of peak subsidence estimated by maximum accumulation rate, (3) correlate estimated peak subsidence with the first appearance of arkose derived from the adjacent denuded Precambrian-cored block, and (4) demarcate synorogenic strata from Permian strata that are postorogenic. Results demonstrate that within New Mexico ARM basins, (1) onset was relatively synchronous, predominantly beginning in early Atokan time; (2) peak subsidence, while potentially younging southward, was relatively coeval in the Middle to Late Pennsylvanian; (3) first occurrence of arkose either predates the period of peak subsidence or is coeval with peak subsidence; and (4) early Permian strata across the study area onlap preexisting faults and folds, and/or form a buttress unconformity with Precambrian basement.

Abstract Recent research in Pennsylvanian-Permian strata of the Fountain Formation adjacent to the Front Range uplift and the Cutler Formation adjacent to the Uncompahgre uplift (Colorado) has resulted in new hypotheses about the climate and tectonics of the Ancestral Rocky Mountains. The Fountain and the Cutler formations are iconic deposits; the thick and coarse-grained nature of these units has been cited for nearly a century as documenting the Ancestral Rockies. Long considered the products of alluvial fan deposition in warm climates, new data support the hypothesis of proglacial deposition for these units, and thus glaciation of the Ancestral Rocky highlands. This bears on our understanding of Late Paleozoic climate in western equatorial Pangaea, as well as global climate at this time. Furthermore, new mapping in the Uncompahgre region indicates substantial onlap and possible burial of Precambrian highlands by Permian strata. The argument for Permian subsidence of the uplift emanates from the hypothesis that Unaweep Canyon, which bisects the Uncompahgre Plateau (paleo Uncompahgre uplift) originated as a Permian paleolandscape. This trip will include visits to (1) the Fountain and Cutler formations to discuss and debate the sedimentologic origin(s) of these deposits, and (2) Unaweep Canyon to examine evidence for both a possible Paleozoic age and glacial origin for this canyon, and its late Cenozoic history as a former stream course of the ancestral Gunnison River.

Abstract The continental interior platform of the United States is that part of the North American craton where a thin veneer of Phanerozoic strata covers Precambrian crystalline basement. N- to NE-trending and W- to NW-trending fault zones, formed initially by Proterozoic/Cambrian rifting, break the crust of the platform into rectilinear blocks. These zones were reactivated during the Phanerozoic, most notably in the late Palaeozoic Ancestral Rockies event and the Mesozoic-Cenozoic Laramide orogeny — some remain active today. Dip-slip reactivation can be readily recognized in cross section by offset stratigraphic horizons and monoclinal fault-propagation folds. Strike-slip displacement is hard to document because of poor exposure. Though offset palaeochannels, horizontal slip lineations, and strain at fault bends locally demonstrate strike-slip offset, most reports of strike-slip movements for interior-platform faults are based on occurrence of map-view belts of en echelon faults and anticlines. Each belt overlies a basement-penetrating master fault, which typically splays upwards into a flower structure. In general, both strike-slip and dip-slip components of displacement occur in the same fault zone, so some belts of en echelon structures occur on the flanks of monoclinal folds. Thus, strike-slip displacement represents the lateral component of oblique fault reactivation; dip-slip and strike-slip components are the same order of magnitude (tens of metres to tens of kilometres). Effectively, faults with strike-slip components of displacement act as transfers accommodating jostling of rectilinear crustal blocks. In this context, the sense of slip on an individual strike-slip fault depends on block geometry, not necessarily on the trajectory of regional σ 1 . Strike-slip faulting in the North American interior differs markedly from that of southern and central Eurasia, possibly because of a contrast in lithosphere strength. Weak Eurasia strained significantly during the Alpine-Himalayan collision, forcing crustal blocks to undergo significant lateral escape. The strong North American craton strained relatively little during collisional-convergent orogeny, so crustal blocks underwent relatively small displacements.

Abstract The Ancestral Rocky Mountains were intracratonic block uplifts that formed in Colorado and the surrounding region during Pennsylvanian time. Their development was related to the collision suturing of North America with South America-Africa, which also resulted in the Ouachita-Marathon orogeny. In Early Pennsylvanian time, suturing was taking place only in the Ouachita region, and foreland deformation took place largely in the midcontinent. By Middle Pennsylvanian time, the length of the active suture zone had increased, extending from the Ouachita region to the Marathon region. At this same time, deformation of the craton also increased in intensity and in areal extent, culminating in the Ancestral Rocky Mountains. In Late Pennsylvanian time, suturing was taking place only in the Marathon region, and cratonic deformation decreased areally and spread southward into New Mexico and western Texas and westward into the Cordillera miogeocline. The Ancestral Rocky Mountains and related features in a broad area of the western United States were formed while an irregularly bounded peninsula of the craton (including the transcontinental arch) was pushed northward and northwestward by the progressive collision suturing of North America and South America-Africa. This intraplate deformation was, in some respects, like the Cenozoic deformation of Asia in response to the collision with India.

Abstract The Middle Pennsylvania!) Minturn Formation and the Pennsylvanian-Permian Sangre de Cristo Formation of the northern Sangre de Cristo Range form a thick progradational sequence of coarse clastic sediments. These sediments were deposited along the western margin of the central Colorado trough during uplift of the late Paleozoic Uncompahgre highland, a major structural and topographic feature of the Ancestral Rocky Mountains. The Minturn is composed mostly of sandstone and shale deposited by fan deltas that prograded into the central Colorado trough. The Minturn of the northern Sangre de Cristo Range is divisible into a turbidite-bearing facies, a limestone-bearing facies, and a redbed facies. The turbidite-bearing facies is interpreted as deposits of fan deltas that prograded onto the sea bottom below wave base. The limestone-bearing facies is interpreted as deposits of fan deltas that prograded onto a shallow sea bottom above wave base. In this facies, sandstones containing deltaic foresets overlie thin shallow marine limestones and are considered diagnostic of that facies. Both facies contain thick intervals of sandstone and shale interpreted as deltaic and alluvial deposits. Where it onlaps the Uncompahgre highland, the lower part of the Minturn Formation contains quartzose and arkosic redbeds of probable alluvial origin. The continental Sangre de Cristo Formation conformably overlies the Minturn Formation basinward, but it unconformably overlies the Minturn and Precambrian basement near the Uncompahgre highland. The Sangre de Cristo contains two facies. A sandstone facies consists of fining-upward cycles of red conglomeratic sandstone and siltstone interpreted as braided stream deposits on distal parts of alluvial fans. A conglomerate facies (Crestone Conglomerate Member) consists of poorly sorted conglomerates interpreted as debris flow and mudflow deposits and sorted conglomerate and sandstone interpreted as streamflow and sheetflow deposits on proximal parts of alluvial fans. The fan deltas and alluvial fans that deposited the Minturn and Sangre de Cristo formations probably developed in response to faulting and uplift of the Uncompahgre highland. The highland rose at least three times from Middle Pennsylvanian to Early Permian time. Later, during the Laramide orogeny, the faulted boundary between the highland and the central Colorado trough was destroyed by thrusting. The boundary faults were not strike-slip, as shown by the distribution of clasts in conglomerates directly downstream from identical Precambrian rocks that represent part of the highland.