Carbonate flat-pebble conglomerate is an important component of Precambrian to lower Palaeozoic strata, but its origins remain enigmatic. The Upper Cambrian to Lower Ordovician strata of the Snowy Range Formation in northern Wyoming and southern Montana contain abundant flat-pebble conglomerate beds in shallow-water cyclic and non-cyclic strata. Several origins of flat-pebble conglomerate are inferred for these strata. In one case, all stages of development of flat-pebble conglomerate are captured within stormdominated shoreface deposits of hummocky cross-stratified (HCS) fine carbonate grainstone. A variety of synsedimentary deformation structures records the transition from mildly deformed in situ stratification to buckled beds of partially disarticulated bedding to fully developed flat-pebble conglomerate. These features resulted from failure of a shoreface and subsequent brittle and ductile deformation of compacted to early cemented deposits. Failure was induced by either storm or seismic waves, and many beds failed along discrete slide scar surfaces. Centimetre-scale laminae within thick amalgamated HCS beds were planes of weakness that led to the development of platy clasts within partly disarticulated and rotated bedding of the buckled beds. In some cases, buckled masses accelerated downslope until they exceeded their internal friction, completely disarticulated into clasts and transformed into a mass flow of individual cm-to dm-scale clasts. This transition was accompanied by the addition of sand-sized echinoderm-rich debris from local sources, which slightly lowered friction by reducing clastclast interactions. The resulting dominantly horizontal clast orientations suggest transport by dense, viscous flow dominated by laminar shear. These flows generally came to rest on the lower shoreface, although in some cases they continued a limited distance beyond fairweather wave base and were interbedded with shale and grainstone beds. The clasts in these beds show no evidence of extensive reworking (i.e. not well rounded) or condensation (i.e. no rinds or coatings). A second type of flat-pebble conglomerate bed occurs at the top of upward-coarsening, metre-scale cycles. The flat-pebble conglomerate beds cap these shoaling cycles and represent either lowstand deposits or, in some cases, may represent transgressive lags. The clasts are well rounded, display borings and have iron-rich coatings. The matrix to these beds locally includes glauconite. These beds were considerably reworked and represent condensed deposits. Thrombolites occur above the flat-pebble beds and record microbial growth before initial transgression at the cycle boundaries. A third (2004) 51, 973-996 Ó 2004 International Association of Sedimentologists 973 type of flat-pebble conglomerate bed occurs within unusual metre-scale, shaledominated, asymmetric, subaqueous cycles in Shoshone Canyon, Wyoming. Flat-pebble beds in these cycles consist solely of clasts of carbonate nodules identical to those that are in situ within underlying shale beds....
Summary In this integrated study using resistivity images, conventional openhole logs, and core data from a Middle Eastern reservoir, abundance and geometric configuration of bedded and nodular evaporite have been studied to help distinguish which nodular forms of evaporite may be related to a permeability suppression. Several logs have been calculated from the resistivity image log to quantify nodular evaporite and help predict the presence of corresponding core facies well. Compared with thin-section description, most samples of nodular evaporite were exhibiting fine-scale cementation as well, and their permeability was suppressed compared with samples with rare or no fine-scale cementation in thin sections.
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