Abstract:

In the study area, southeast of Zara in the Sivas Basin, the Lower-Middle Miocene marine Karacaören Formation consists of a succession of marine sedimentary rocks deposited within two trangressive-regressive cycles. The present study focuses on the facies characteristics, cyclic nature, and significance of the basin-wide distribution of a 100-m-thick gypsiferous sedimentary package overlying an otherwise transgressive segment of the lower sequence, within a marine setting. The typical lithologic expression of the studied sedimentary interval comprises generally fossiliferous mudstones, rare algal limestones and nodular gypsum. Detailed sedimentological observations, combined with palaeontological evidence, indicate that the mudstone intervals were deposited in a subtidal to lower intertidal setting, while the algal limestones represent intertidal areas. Nodular gypsum and associated rare reddish-greenish clays, which constitute nearly half of the studied sequence, were probably deposited within a coastal sabkha environment. The internal architecture of the studied interval is characterized by some fifteen shallowing-upwards (i.e., progradational) cycles. Each cycle typically has a basal mudstone interval, locally overlain by an algal carbonaceous mudstone/limestone, and an upper nodular gypsum (with or without reddish-greenish clays) interval. These cycles probably correspond to parasequences which were driven by high-frequency changes in relative sea-level. The lateral distribution pattern of the studied evaporitic interval implies that conditions giving rise to evaporites prevailed over the entire central and eastern parts of the Sivas Basin, and that the local palaeogeography at that time was characterized by widespread coastal sabkhas to the south, and a reefal-to-muddy shelf to the north.

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