The Early and Middle Proterozoic rocks in the Needle Mountains include three distinct rock sequences (1) multiply deformed bimodal metavolcanic rocks, related sedimentary rocks, and plutonio units, all metamorphosed to medium grade; (2) multiply deformed clastic sedimentary rocks metamorphosed to low grade; and (3) weakly foliated to unfoliated plutonic rocks. Although many important questions remain unresolved, work by a number of individuals suggests the following history. Mafic and silicic volcanics, sediments, and intrusives of the Irving Formation and Twilight Gneiss accumulated at least 1,760 Ma and were multiply deformed and metamorphosed shortly after accumulation. Small post-tectonic plutons of the Tenmile and Bakers Bridge Granites invaded the metavolcanic sequence between 1,680 and 1,700 Ma. The timing of deposition of the conglomerates, clean quartzites, and pelites of the Vallecito Conglomerate and the Uncompahgre Formation with respect to events in the metavolcanic sequence is poorly constrained, largely because shear zones lie along crucial contacts. Recent work suggests that much if not all of this clastic sedimentary sequence may be parautochthonous and was probably deposited sometime after 1,680 to 1,700 Ma. Despite uncertainties in timing of deposition, it is clear that a second event of deformation and metamorphism affected the region, producing polyphase features in both the metavolcanic sequence and the clastic sedimentary sequence, as well as a weak foliation in the Tenmile Granite. The Eolus Granite and related intrusions, ranging in age from 1,320 to 1,450 Ma, crosscut all structural features in the region and provide a minimum age limit for the latest deformation in the region. This younger event is recorded nowhere else in Colorado but may correlate with events in northern New Mexico.

Abstract Paleozoic strata of the Southern Rocky Mountains and Colorado Plateau overlie a structurally complex basement of metamorphic, igneous, and metasedimentary rocks. The basement complex was completed in Proterozoic time by major continental-scale wrench faulting that produced a northwesterly trending swarm of faults (the Olympic-Wichita Lineament of Baars, 1976) and a northeasterly set (the Colorado Lineament of Warner, 1978). The apparently conjugate sets of fractures intersect in the heart of the Paradox Basin of the eastern Colorado Plateau. Precambrian sense of displacement was dextral along the Olympic-Wichita trend, at least on the Colorado Plateau (Baars and Stevenson, 1982), and sinistral along the Colorado Lineament (Warner, 1978). Thus, sigma one was directed in a north-south orientation (Fig. 1). Age of the northwesterly faults is bracketed to 1.72 Ga to 1.46 Ga (Baars and Ellingson, 1984); younger granitic intrusives (-1.46 Ga) invade older metamorphics, metasediments, and faults without discretion in the San Juan Mountains. Warner (1978) dated the northeast set at about 1.7 Ga. Metasedimentary rocks (quartzites and metapelites) of the Uncompahgre Formation are juxtaposed with the metamorphic basement (-1.78 Ga) along the fault blocks in the San Juan Mountains of southwest Colorado. The shape and location of the sedimentary basin for these Proterozoic rocks is enigmatic, but stratigraphic similarities and speculative dating would relate the metasediments with the Mazatzal Group of central Arizona and the Ortega Group of north-central New Mexico (C. W. Harris, personal communication, 1987). Ritzma (1987) argued that the age of the Uinta Mountain Group of northern Utah is

Abstract The Paradox basin, about 360 miles long in northwesterly trend, and 180 miles wide, subsided obliquely across the high southeast shelf of the Cordilleran geosyncline during mid-Pennsylvanian time after an earlier Pennsylvanian history of shelf deposition of red clastics to form the Molas formation, and gray marine shale and limestone to form the Pinkerton Trail formation. The early autogeosynclinal subsidence of this intracratonic parageosyncline allowed the deposition of the Paradox formation comprising major cyclic deposits of normal marine, penesaline, and saline strata, and simultaneous emplacement of medium-to coarse-grained clastics from the Uncompahgre-San Luis-Penasco highland on the northeast and east, and fine- to medium-grained clastics from the Zuni-Defiance-Kaibab-Emery positives on the south, southwest, west, and northwest. This restricted basin of evaporite deposition underwent rigorous aridity cyclically interspersed with what appear to have been humid conditions, while normal marine waters entered the basin episodically to cyclically through sedimentational passes or marine accessways from the Sonoran geosyncline on the south and the Cordilleran geosyncline on the west. Increased tectonic intensity in late Pennsylvanian led to sharp subsidence of the northeast flank of the Paradox autogeosyncline, to form the Uncompahgre taphrogeosyncline, into which were dumped many cubic miles of coarse arkosic clastics from the sharply uplifted Uncompahgre highland. These clastics interfinger southwestward with normal marine shale and carbonates of the Honaker Trail formation, deposited as the basin became thoroughly ventilated via additional marine accessways across the shelves of the basin. High on the Paradox-Uncompahgre southwestern shelf, Coconino-type quartzose clastics and Supai-type fine clastics moved into the basin from westerly and southerly directions, which, combined with influx of Uncompahgre-San Luis-Penasco clastics from the northeast and east, drove the seas from the area during latest Pennsylvanian to early Permian time. The interfingering of invading clastics, combined with the generation of organic deposits along a broad zone of the southwest sedimentational shelf parallel with the strike of the Paradox basinal axis where thick evaporites were formed, created the facies gradations that make the Paradox basin a classic area for the study of Pennsylvanian stratigraphic and sedimentologic complexities.

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 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.