Autochthonous, doubly terminated, euhedral megaquartz crystals, as well as overgrowths on detrital quartz grains, are present throughout a gypsum- and anhydrite-bearing Pleistocene sabkha dolomite accumulation from the Arabian Gulf. The siliceous precipitates, whose source was probably recycled biogenic silica, formed within meters of the sediment surface. These Quaternary precipitates provide the first modern analog for euhedral megaquartz crystals, a relatively common constituent in ancient evaporite-bearing tidal-flat carbonate strata. In the Quaternary dolomites studied, megaquartz is present as individual crystals and clusters of crystals, as well as overgrowths on detrital quartz grains. Individual crystals commonly attain lengths of 1 mm, and many show shallow dolomite mold impressions on their crystal facies and/or partly, or completely, engulf dolomite rhombohedra. The common occurrence of dolomite molds within the quartz crystals indicates that precipitation of the quartz took place either concurrently with, or more likely, after dolomitization of the host carbonates. Additionally, the oft-cited association of authigenic megaquartz and evaporite minerals, plus the existence of void spaces into which the quartz precipitated, imply a causal relationship between dissolution of the evaporites and/or dolomites and precipitation of the megaquartz. Thus, these findings demonstrate that authigenic megaquartz crystals as well as overgrowths on detrital quartz can originate under near-surface temperatures and pressures, and very early in the diagenetic history of evaporite-associated carbonate strata.

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