Joseph L. Allen

Fallen arches: Dispelling myths concerning Cambrian and Ordovician paleogeography of the Rocky Mountain region

High-resolution sedimentologic, biostratigraphic, and stable isotope data from numerous measured sections across Colorado reveal a complex architecture for lower Paleozoic strata in the central Cordilleran region. A lack of precise age control in previous studies had resulted in misidentification and miscorrelation of units between separate ranges. Corrections of these errors made possible by our improved data set indicate the following depositional history. The quartz-rich sandstone of the Sawatch Formation was deposited during onlap of the Precambrian erosion surface in the early Late Cambrian. The overlying Dotsero Formation, a regionally extensive carbonate- and shale-rich succession records blanket-like deposition with only minor facies changes across the state. An extremely widespread, meter-scale stromatolite bed, the Clinetop Bed, caps the Dotsero Formation in most areas. However, a latest Cambrian erosional episode removed 9–11 m of the upper Dotsero Formation, including the Clinetop Bed, from just east of the Homestake shear zone in the Sawatch Range eastward to the Mosquito Range. The overlying Manitou Formation differs in character, and thus in member stratigraphy, on the east vs. west sides of the state. These differences were previously interpreted as the result of deposition on either side of a basement high that existed within the Central Colorado Embayment or Colorado “Sag,” a region of major breaching across the Transcontinental Arch. This paleogeographic reconstruction is shown herein to be an artifact of miscorrelation. Biostratigraphic data show that the northwestern members of the Manitou Formation are older than the members exposed in the southeastern part of the state and that there is little or no overlap in age between the two areas. This circumstance is the result of (1) removal of older Manitou Formation strata in the southeast by an unconformity developed during the Rossodus manitouensis conodont Zone, and (2) erosion of younger Manitou strata in central and western Colorado along Middle Ordovician and Devonian unconformities. Deciphering these complex stratal geometries has led to invalidation of long-held views on western Laurentian paleogeography during the Cambrian and earliest Ordovician, specifically the existence of the Colorado Sag and a northeast-trending high within the sag that controlled depositional patterns on either side. The mid- Rossodus uplift and resultant unconformity eliminated any and all Upper Cambrian and Lower Ordovician deposits in southern Colorado and northern New Mexico, and thus their absence should not be misconstrued as evidence for earlier nondeposition in this region. Lithofacies distribution patterns and isopach maps provide no evidence that highlands of the Transcontinental Arch existed in Colorado prior to the mid- Rossodus age uplift event. In fact, regional reconstructions of earliest Paleozoic paleogeography along the entire length of the purported Transcontinental Arch should be reevaluated with similarly precise biostratigraphic data to reconsider all potential causes for missing strata and to eliminate topographic elements not supported by multiple stratigraphic techniques. This study illustrates how seriously paleogeographic reconstructions can be biased by the presumption that missing strata represent periods of nondeposition rather than subsequent episodes of erosion, particularly in thin cratonic successions where stratigraphic gaps are common and often inconspicuous.

Structural geometry and thermal history of pseudotachylyte from the Homestake shear zone, Sawatch Range, Colorado

Pseudotachylytes are frictional melts that can be formed either during meteorite impact (Grieve, 1975; Bischoff, 1982; Reimold and Colliston, 1994; Spray and Thompson, 1995), or during coseismic faulting (Sibson, 1975; Cardwell et al., 1978; Spray, 1992; Swanson, 1992). Pseudotachylyte in the Homestake shear zone (HSZ) crops out as dark, aphanitic veins that have a distinct linear mapand outcrop-scale geometry that suggests they are a product of coseismic faulting. Pseudotachylyte was first recognized in Colorado during early geologic mapping that defined the HSZ (Tweto and Sims, 1963; Tweto, 1974) and was first described by Allen (1994) from isolated exposures. Subsequent detailed mapping of pseudotachylyte veins presented herein has lead to the recognition of a systematic mapscale distribution of pseudotachylyte-bearing fault zones. This field trip will examine selected well-exposed outcrops that form a very small part of an interconnected, > 25 km system of pseudotachylyte fault veins that formed during strikeslip faulting. The fault veins are hosted within isoclinally folded, early Proterozoic biotite gneiss and pelitic schist within the HSZ in the northeastern Sawatch Range of central Colorado. Field trip objectives include: (1) demonstration of meterto kilometer-scale along-strike continuity of pseudotachylyte fault veins; (2) examination of evidence for pre-existing, foldrelated structural controls on the geometry of the overprinting coseismic rupture that produced pseudotachylyte; (3) discussion of petrologic and mineralogic evidence for a high-T origin of the melt; and (4) presentation of preliminary results of a field test of a recently proposed pseudotachylyte geothermometer (O’Hara, 2001).

Seismogenic structure of a crystalline thrust fault: fabric anisotropy and coeval pseudotachylyte–mylonitic pseudotachylyte in the Grizzly Creek Shear Zone, Colorado

Abstract Field and microstructural observations from the Proterozoic Grizzly Creek Shear Zone suggest that crustal-scale fabric anisotropy exerted a significant control on earthquake rupture propagation during deformation at mid-crustal depths. The shear zone developed in amphibolite-facies supracrustal gneisses and granitoids, and consists of a 0.4–0.7 km-wide zone of high-strain rocks with foliation transposed to 256°/51°NW and top-to-the-south kinematics. The shear zone is overprinted by hundreds of veins of pseudotachylyte, mylonitic pseudotachylyte and ultramylonite. Field observations and whole-rock geochemical data suggest that pseudotachylyte fault veins formed as a result of first-generation rupture through intact rock. Pseudotachylytes are preferentially localized in as many as nine decametre-scale rupture zones dispersed across the width of the shear zone, concordant to foliation. We present a conceptual model for the asymmetric development of anisotropic fabric in a thrust-related fault zone in crystalline metamorphic rocks. Progressive tectonic exhumation of hanging wall rocks during thrusting results in the development of a crustal-scale anisotropic fabric that provides a preferentially weakened zone that could accommodate the propagation of earthquake ruptures from the seismogenic zone into the middle crust.

Timing, style, and significance of Cambrian through Laramide brittle reactivation along the Proterozoic Homestake shear zone, Colorado mineral belt

Detailed mapping and stratigraphic studies at the intersection of the Proterozoic Homestake shear zone with the early Paleozoic section exposed on the margins of the Laramide Sawatch anticline in central Colorado provide limits on the timing and magnitude of brittle reactivation during Phanerozoic time. An episode of displacement rooted within two distinct ductile branches of the Homestake shear zone in the northeastern Sawatch Range generated an 8-km-wide, Late Cambrian fault block that had 4–20 m of paleotopographic relief prior to depositional onlap by sandstones of the Sawatch Formation. An episode of up-to-south displacement (<30 m) subsequently occurred along the southern part of the shear zone in the northeastern Sawatch Range and in the western Sawatch Range during deposition of the Lower Ordovician Manitou Formation. A third episode of reactivation during Early to Middle Ordovician time (post-Manitou Formation, pre-Harding Sandstone) resulted in renewed, decameter-scale uplift of the Late Cambrian fault block in the northeastern Sawatch Range. A fourth episode of reactivation during Late Cretaceous Laramide deformation produced localized strike-slip displacement along brittle faults on the northeastern flank of the Sawatch Range. The three early Paleozoic episodes of reactivation occurred on a cratonic platform and are genetically linked to extension and thermal uplift during intrustion of a suite of bimodal, rift-related plutonic rocks (Iron Hill and Wet Mountains intrusive rocks). They were emplaced ∼135 km south of the Homestake shear zone along the Cimarron-Red Rocks fault and Apishapa fault system. The reactivation history model proposed herein differs from some previous interpretations that cumulatively suggest at least eleven episodes of Phanerozoic reactivation, including relatively large-magnitude late Paleozoic displacement.