Coastal-Marine Accumulative Forms in Neogene Carbonate Sediments with the Transcaspian Region as Example

The relief of high limestone plateaus of the Transcaspian region (Ustyurt, Steppe Mangyshlak, and Krasnovodsk) is leveled and primarily tectonic: ridges and depressions (tens and hundreds of kilometers in length and tens, rarely a few hundred meters in amplitude) are composed of gentle (10–40) positive and negative deformations of the Neogene cover. The plateaus are bounded by ledges (chinoks). Against the background of the gentle hilly topography, one can observe smaller but sufficiently long (tens of kilometers) groups of arc-shaped parallel limestone ridges (2–8 m high) that crosscut large morphological irregularities. All topographic forms on the plateau resulted from a prolonged (many millions years) weathering and arid denudation. Therefore, insignificant differences of stability appeared in limestones armoring the plateau. Analysis of the composition and structure of limestones in linear arc-shaped ridges of ledges and the plateau surface showed that the limestones represent an extremely shallow-water facies of shelf sediments. The arc-shaped limestone ridges on the plateau are exposed, partially modified, accumulative, and biogenic-accumulative forms of bottom relief. Similar accumulative forms coupled with biogenic buildups are known in nontropic carbonate shelves of recent seas. The finding of coastal-marine forms on the limestone plateau surface suggests a shallow-water sedimentation regime in Neogene basins persisting over short time spans. Study of such forms is of interest for the refinement of water depths, wave–wind regime, and currents in Neogene basins, as well as for prospecting for oil–gas fields and placer accumulations.