A shallow marine clinoform system in limestones (Paleocene/Eocene Jafnayn Formation, Oman): geometry, microfacies, environment and processes

摘要

This is the first report of Cenozoic shallow marine limestone clinoforms from Oman in an outcrop of exceptional exposure. The depositional system displays a few hundred well-developed foreset beds/clinobeds, representing a paleoslope of15 degrees (post-compaction), occurring in a 3-5m-thick stratigraphic interval. In the outcrop, foreset beds can be followed along for approximately 20m. The foreset beds are composed of packstones to floatstones. They pass into very short, minimally developed bottomset beds with a base-discordant relation (downlap). Directly below the bottomset beds is a marly limestone bed (wackestone to floatstone). Bedding of the topset beds is largely obliterated by bioturbation, causing nodular bedding in the topset segment and in an overlying red algae-bearing unit, consisting of floatstones to rudstones. The clinobeds represent cross-beds within an overall aggradational sequence. We use the geometry of the clinoform systems as a platform level. Although the foreset process had been repeated several hundred times, neither the steepness of the paleoslope nor the length/height of the foreset beds changed, suggesting a very gentle slope as on a shelf and not as on a ramp. Because of local evidence for synsedimentary extensional faulting, we assume that such deformation caused the creation of the initial intraplatform slope. Dasycladacean algae indicate that deposition ensued in the photic zone. Above the red algae unit, alveolinid wackestones to floatstones accumulated. The observed microfacies change from the marly limestone to the alveolinid limestone indicates that the studied clinoform system developed within a shallowing-up succession. In the same direction, water energy and coastal influences increased. The microfacies types are compatible with a lagoon setting on a shelf with open circulation. Erosion of the red algae may have been due to moving and abrading skeletal grains, driven by tidal and rip currents and/or boring organisms. Tidal and rip currents are suggested to have shed the red algae fragments and other carbonate debris onto the depositional slope. In the absence of early marine cements, slope stability is attributed to the moderate paleoslope angle, different grain shapes and possibly to encrustation and binding of sediment by red algae. The presence of Ranikothalia sp. demonstrates that the succession dates from Paleocene to lowermost Eocene. Similarly, the red algae Distychoplax biserialis indicates a Paleocene to Lower Eocene age.