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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.earscirev.2005.07.012 Byline: John H. Stewart Keywords: eolian deposits; Neoproterozoic; San Bernardino, Mountains, California, USA Abstract: Strata interpreted to be eolian are recognized in the Neoproterozoic Big Bear Group in the San Bernardino Mountains of southern California, USA. The strata consist of medium- to large-scale (30 cm to >6 m) cross-stratified quartzite considered to be eolian dune deposits and interstratified thinly laminated quartzite that are problematically interpreted as either eolian translatent climbing ripple laminae, or as tidal-flat deposits. High index ripples and adhesion structures considered to be eolian are associated with the thinly laminated and cross-stratified strata. The eolian strata are in a succession that is characterized by flaser bedding, aqueous ripple marks, mudcracks, and interstratified small-scale cross-strata that are suggestive of a tidal environment containing local fluvial deposits. The eolian strata may have formed in a near-shore environment inland of a tidal flat. The Neoproterozoic Big Bear Group is unusual in the western United States and may represent a remnant of strata that were originally more widespread and part of the hypothetical Neoproterozoic supercontinent of Rodinia. The Big Bear Group perhaps is preserved only in blocks that were downdropped along Neoproterozoic extensional faults. The eolian deposits of the Big Bear Group may have been deposited during arid conditions that preceded worldwide glacial events in the late Neoproterozoic. Possibly similar pre-glacial arid events are recognized in northern Mexico, northeast Washington, Australia, and northwest Canada. Author Affiliation: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA, 94025 United States

Faults in sandstone are frequently composed of two classes of structures: (1) deformation bands and (2) joints and sheared joints. Whereas the former structures are associated with cataclastic deformation, the latter ones represent brittle fracturing, fragmentation, and brecciation. We investigated the distribution of these structures, their formation, and the underlying mechanical controls for their occurrence along the Moab normal fault in southeastern Utah through the use of structural mapping and numerical elastic boundary element modeling. We found that deformation bands occur everywhere along the fault, but with increased density in contractional relays. Joints and sheared joints only occur at intersections and extensional relays. In all locations, joints consistently overprint deformation bands. Localization of joints and sheared joints in extensional relays suggests that their distribution is controlled by local variations in stress state that are due to mechanical interaction between the fault segments. This interpretation is consistent with elastic boundary element models that predict a local reduction in mean stress and least compressive principal stress at intersections and extensional relays. The transition from deformation band to joint formation along these sections of the fault system likely resulted from the combined effects of changes in remote tectonic loading, burial depth, fluid pressure, and rock properties. In the case of the Moab fault, we conclude that the structural heterogeneity in the fault zone is systematically related to the geometric evolution of the fault, the local state of stress associated with fault slip, and the remote loading history. Because the type and distribution of structures affect fault permeability and strength, our results predict systematic variations in these parameters with fault evolution. Keywords: deformation band, joint, Moab fault, mechanics, stress, sheared joint.

Most new oil and gas reservoirs are discovered in sands and weak sandstones, and the petroleum industry has therefore generated an extensive amount of data from laboratory tests on these materials. Some of these data are reexamined in this paper within the context of recent soil mechanics research. Of key importance to petroleum engineers is the compressibility of the sands, and this is shown to be related both to the current density and stress level of the soil and to the location of its normal compression line. When present, the cement bonds between the particles appear to be weaker than the particles themselves, so that the major yield point during compression is still associated with breakage of the particles rather than of the cement bonds. The paper also examines the behavior of the sands and sandstones in shearing, allowing a more general understanding of the mechanics of the materials to be developed. CE Database subject headings: Sand; Sandstone; Triaxial tests; Oil recovery; Stiffness; Strength; Compressibility.

Late Jurassic sandstones of the shallow-marine Sundance Formation contain authigenic chlorite minerals that occur as rosette-like pore fillings of interstratified berthierine/chamosite (B-C) and honeycomb-like pore linings of corrensite and discrete chlorite. B-C is nearly ubiquitous in Sundance sandstones, but is absent near the top of the formation, whereas corrensite and discrete chlorite were detected only in uppermost Sundance sandstones, within 4 m of the contact with the overlying non-marine Morrison Formation. Glauconite grains are common and occur as laminae along bedding planes and cross-beds, indicating reworking and deposition as clasts in tidally influenced regimes. The mineralogical, chemical, and morphological properties of the B-C and corrensite indicate that they are authigenic and formed during burial diagenesis from precursor minerals, odinite in the case of B-C, and saponite in the case of corrensite and discrete chlorite. Odinite has been recognized in numerous shallow-marine sands of the Holocene verdine facies, and the shallow-marine conditions associated with Sundance deposition would have been ideal for odinite formation. Saponite commonly forms in aeolian and evaporitic environments, implying that the saponite precursor to corrensite and discrete chlorite formed in uppermost Sundance sands that were exposed to an influx of oxidizing groundwater following regression of the Sundance sea. X-ray diffraction (XRD) indicates that the proportion of 7 [Angstrom] B layers in B-C ranges from 5 to 28%, and chemical analysis by scanning electron microscope-energy dispersive X-ray spectrometry (SEM-EDS) indicates positive correlation between %B and Fe/(Fe + Mg). The polytype of the B-C is Ibb, and the corrensite and discrete chlorite are disordered IIb. These are the expected polytypes and %B for sandstones exposed to burial diagenetic conditions of 3000-4000 m and temperatures 90-120[degrees]C.