PALEOENVIRONMENTAL DISTRIBUTION OF ORDOVICIAN CALCIMICROBIAL ASSOCIATIONS IN THE TARIM BASIN, NORTHWEST CHINA

Abstract

The stratigraphic and facies distribution of 20 calcimicrobial genera (including calcified cyanobacteria and associated problematic calcified microfossils) are reported for the entire Ordovician succession in the Tarim Basin in northwestern China based on examination of drill cores and 8500 thin sections from 64 wells from the Tabei, Bachu, Tazhong, and Tadong uplifts. A total of ten calcimicrobial associations are recognized in the Lower to Upper Ordovician based on taxonomic composition and distribution within four paleoenvironment types: reef (marginal and patch reef), open platform/bank (marginal and patch bank), lagoon, and tidal flat. The temporal distribution of the calcimicrobial genera closely follows changes in sedimentary environments; an extensive literature survey reveals similar relationships in much of the Paleozoic and Mesozoic. Based on their paleoenvironmental preferences, calcimicrobes can be classified into five paleohabitat types: (1) reef-adapted (Acuasiphonoria, Razumovskia, Phacelophyton , Gomphosiphon, Epiphyton, Renalcis, and Izhella); (2) open platform/bank-adapted (Subtifloria and Bevocastria); (3) both reef and open platform/bank-adapted (Bija, Apophoretella, Rothpletzella, and Wetheredella); (4) lagoon-adapted (Hedstromia, Cayeuxia, Zonotrichites, Ortonella, and Garwoodia), and (5) not only reef and open platform/bank-adapted but also tolerant of tidal flat conditions (Girvanella and Proaulopora). The occurrences of these calcimicrobes in strata not only can indicate ancient sedimentary facies but also can reveal paleoecological parameters of ancient seas, such as nutrient levels (e.g., N and P), predation pressure, and sea level, especially in strata absence of other well-studied facies fossils.