FACIES CONTROL ON THE COMPOSITION OF SERPUKHOVIAN AND EARLY BASHKIRIAN FORAMINIFERAL ASSEMBLAGES IN THE MIDDLETIEN-SHAN MOUNTAINS, CENTRAL ASIA
Serpukhovian-lower Bashkirian deposits are widely developed in the Middle Tien-Shan Mountains of Uzbekistan and adjacent countries of Central Asia. These deposits formed in a sedimentary basin exhibiting four distinctive facies that differ in foraminiferal diversity and population density. The facies types, named for mountain ranges containing representative sections, are called 1) Talassic, for inner shelf, shallow- water marine carbonates; 2) Ugamic, for carbonaceous deposits accumulating on an open, shallow-water, outer carbonate shelf platform; 3) Karzhantauic, for interbedded volcaniclastics and shallow-water marine carbonates deposited on a eroded surface; and 4) Paltauic, for basinal beds containing thin-bedded, graded and laminated organic limestones and interbedded turbidites. A statistical program (Sorenson's Coefficients of Species Similarity) was used to compare assemblages in eight foraminiferal zones from coeval fades across the basin. Highest similarity coefficients occur in the early Serpukhovian and are probably related to a marine transgression that flooded the basin. Regression and volcaniclastic sedimentation account for lower coefficients in the remainder of the Serpukhovian. Increased foraminiferal diversity and abundance in the earliest Bashkirian were probably caused by the opening of new connections to adjacent Paleotethyan basins only to be followed by more restricted environmental conditions and lower similarity coefficients later in the early Bashkirian.
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