THE DEPOSITIONAL ENVIRONMENT OF MARINE EVAPORITES: A CASE FOR SHALLOW, CLASTIC ACCUMULATION

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ABSTRACT

The Upper Miocene Solfifera Series of Sicily contains very coarse, massive selenite; parallel laminated gypsum; wavy, stromatolitic laminated gypsum; planar- and ripple cross-laminated gypsum-skeletal calcite sandstones; flat-pebble and fining-upward gypsum conglomerates; and nodular gypsum. The assemblage of sedimentary features indicates deposition—much of it detrital—in a shallow lagoon-littoral flat complex.

Using modern tidal flats as a guide, we interpret the laminations to form when onshore storms flood the shore-line area with sediment-charged seawater. Algal mats bind the newly deposited gypsiferous layer. Flat-pebble conglomerates are formed when storm waves rip up mudcracked, algally-bound laminated sediment. The gypsum nodules are similar to the anhydrite nodules of the modern Persian Gulf sabkhas. They form within sub-aerially exposed skeletal sand just above the groundwater table. The gypsum sandstones accumulated periodically in very shallow shoals formed by wind-driven currents. Large selenite crystals grew in increments during deposition, as indicated by flat-topped pockets of gypsum sand between selenite crystals, selenite crystals draped by algal laminations, and intraformational conglomerates of selenite fragments.

We believe this model of very shallow strand-line lagoonal accumulation, partly detrital and partly diagenetic, may apply to the early stages of many ancient marine evaporite deposits.

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