--> --> Remarkable Penecontemporaneous Deformation Features Produced by Seismic Waves in Cambrian Carbonate Deposits, WEestern Colorado, USA

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Remarkable Penecontemporaneous Deformation Features Produced by Seismic Waves in Cambrian Carbonate Deposits, WEestern Colorado, USA

Abstract

Penecontemporaneous deformation features preserved in Upper Cambrian grainstone and flat-pebble conglomerate beds in western Colorado, USA, include slide scarps, thrusted beds, irregular blocks and internally deformed beds. Slide scarps are sharply defined, concave-up surfaces that truncate underlying bedding. Thrusted beds capture movement of a part of a bed onto itself, generally along a moderate to steeply inclined (generally 25°–40°) ramp. Lenses in the hanging wall of these thrusts show fault-bend geometries and mildly deformed bedding that mimic their geometry. Isolated irregular blocks with variable internal structure rest on flat upper bedding surfaces and are similar in composition to the underlying beds. In this case, parts of beds detached, moved up onto, and some distances across, the laterally adjacent undisturbed bed surfaces. Blocks moved both at the sediment-water interface and intrastratally at shallow depths within overlying muddy deposits. Internally deformed beds have large blocks, fitted fabrics of highly irregular fragments, and contorted lamination, which represent heterogeneous deformation, such as brecciation and liquefaction. These deformation structures were triggered by earthquakes, based on the nature of deformation, regional distribution of liquefaction structures, and their relationships to overlying and underlying beds. The earthquakes represent reactivation of Mesoproterozoic, crustal-scale shear zones in the central Rockies during the Late Cambrian. Features produced by initial brittle deformation are unusual relative to most reported seismites, and may represent poorly recognized to unrecognized seismogenic structures in the rock record.