Faunal associations,paleoecology and paleoenvironment of marine Jurassic rocks in the Mae Sot,Phop Phra,and Umphang areas,western Thailand |
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Authors: | Assanee Meesook JinGeng Sha Chotima Yamee Wirote Saengsrichan |
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Institution: | 1.Department of Mineral Resources,Bangkok,Thailand;2.LPS, Nanjing Institute of Geology and Palaeontology,Chinese Academy of Sciences,Nanjing,China |
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Abstract: | We here report a paleoecological analysis and depositional history of the marine Jurassic (Toarcian-early Bajocian) strata
cropping out in the western part of Thailand, based on bivalve assemblages with additional data from ammonites, brachiopods,
and microfossils. Generally, the benthic bivalve facies in most outcrops is rich in infaunal, semi-infaunal and epifaunal
suspension-feeders. Of these, infaunal forms dominate. The diversity of this benthic assemblage was influenced by energy level,
substrate, sedimentation rate, and salinity. Low to intermediate energy levels and rather soft fine-grained siliciclastic
substrate are proposed as factors governing faunal distribution and explaining the greater abundance and diversity of infaunal
than epifaunal suspension-feeders. There were paleoenvironmental changes both in space and time, i.e., from south to north
(Umphang to Mae Sot) and from Early Bajocian to Toarcian. In the Toarcian, most outcrops in Umphang are dominated by benthic
bivalve facies (infaunal, semi-infaunal, and epifaunal associations). This implies warm, shallow water (inner neritic, 50–100
m) and oxygenated conditions except for the Mae Sot area where a deeper setting (outer neritic to possibly upper continental
slope, 50–200 m) with restricted basinal anoxic conditions is favored as indicated by the presence of Bositra. After higher energy conditions in the Toarcian, lower energy conditions with low sediment supplies prevail in the Alenian,
and the Mae Sot area was still a restricted basin. As a result of higher sea levels, the oxygen content in the basin is increased,
resulting in the presence of the ammonites. By the end of the Alenian-early Bajocian, an ammonite-bivalve association (mixed
facies A) and the presence of corals and microfauna (mixed facies B) are dominant but pass upwards to near-shore higher energy
conditions in most areas except for restricted basin in Mae Sot. By the middle Bajocian the environment in all areas had changed
from marine to nonmarine. |
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