Marine derived 87Sr/86Sr in coal, a new key to geochronology and palaeoenvironment: Elucidation of the India-Eurasia and China-Indochina collisions in Yunnan, China

Coal has formed in terrestrial and coastal-marine environments from sub-polar to equatorial regions since the Devonian. It contains detailed long-term records of contemporaneous environment, climate, and subsequent modifications. However, in general, direct chronological information in coal has been sparse. The coal investigated in the present study is from the Mile intermontane basin, Yunnan Province, China, north of an arm of the Mesozoic Tethys Ocean. The coal contains marine geochemical signatures and syngenetic gypsum, common in coastal-marine sediments. The gypsum contains marine-derived Sr and, hence, has geochronological potential. The Sr-87/Sr-86 record (0.708350-0.708591) in the Mile coal agrees with time-calibrated Sr-87/Sr-86 records of marine planktonic foraminifera obtained from core DSDP 588C, 22.25-18.27 Ma (Early Miocene). The peat of the Mile coal was deposited over 4.6 Ma., which possibly is the longest deposition of a coal bed in the world to have been found today, although this duration should include the period of non-peat deposition or erosion if present during the time of the 4.6-Ma. During this period, the regional geological structures were determined by the India-Eurasia collision, which resulted in transform faults with extensive rift structures, including the Mile rhomb-shaped graben. This structural setting enabled the flow of seawater from the South China Sea to reach inland graben structures, including that of the Mile Basin, where peat was deposited. Subsequent deformation caused by the South China-Indochina collision changed the regional structural and geographical-hydrological patterns. This affected the hydrology of the Mile Basin and resulted in its uplift to its present-day elevation of 1350 m. This study is, to our knowledge, the first to use the marine-derived Sr-87/Sr-86 indicator and chronometer in coal. At present, marine-influenced peals generated in coastal salt marshes extend from the Arctic Ocean (Alaska and Siberia) in the north, to Patagonia and New Zealand in the south, while mangrove forests abound in equatorial and low-latitude coastal areas. The Sr-87/Sr-86 record of marine-influenced coal in this study area, provides a key for the determination of age-duration-rate of geological processes in the inland basin, associated with the closure of the Tethys Ocean. Results indicate that this method has potential for providing a temporal framework for geological events and processes in other areas. These may be found near marine shore lines across the globe dating back to the Devonian. Moreover, the Sr-87/Sr-86 signal in marine-influenced coal can be used for correlation with the well-established Sr-87/Sr-86 chronology of marine planktonic foraminifera. In turn, the Sr-87/Sr-86 signal provides a basis for correlating terrestrial records of climate and environment contained in this type of coal and associated sediments, with those of marine sediments, such as those based on delta O-18 in planktonic foraminifera.