Cyclostratigraphy in continental high latitude Central Asia during the Early Jurassic
Abstract
Continuous, Early Jurassic age continental deposits are well exposed in the southern margin of the paleo-high-latitude (50-70° N) Junggar Basin in Central Asia. Constrained by biostratigraphy, Jurassic strata span from the boundary with the Triassic to the Toarcian. High-resolution stratigraphic measurements were carried out in this area and provide a synoptic view of the cyclostratigraphy of North Central Asia during the Early Jurassic. We constructed a scale of lake depth, based largely on grain size, color, sedimentary structures and fossil types termed the depth rank index allowing time series analysis. Depth rank index can be interpreted as a proxy of climate under a stable tectonic setting. Cyclicity is visually evident in both outcrop and the depth rank index in the Lower Jurassic of the southern Junggar Basin including a prominent cycle with a period of 20-40 m, composed of, from bottom to top, shallow lake sandstones with ripples or ripple cross-bedding, floodplain paleosols with abundant roots, and deep lake dark gray mudstones rich in fish scale, clam shrimp, bivalve and insect fossils. It is most simply interpreted as due to lacustrine depth changes paced by the 405 kyr eccentricity cycle (based on average accumulation rate) - the most stable orbital cycle over geological time related to the Venus-Jupiter beat cycle and the most appropriate term for the calibration of Jurassic astronomical time. Additionally, the Early Jurassic in Central Asia is characterized by coal-bearing strata in the lower to middle part, deposits with red beds, caliche nodules, mud cracks and heating and arid loving plant group in the middle to upper part, and then coal-bearing strata again in the uppermost part, indicating climate change from humid to arid then to humid. Paleobotanical and bivalve biostratigraphy suggests the arid climate interval was during the early Toarcian, perhaps related to high atmospheric CO2. Further development of the astronomical time scale of the Early Jurassic in the Central Asia should help place the Central Asia arid event in a global temporal framework.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMPP53D1227F
- Keywords:
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- 1165 Sedimentary geochronology;
- GEOCHRONOLOGYDE: 3270 Time series analysis;
- MATHEMATICAL GEOPHYSICSDE: 4910 Astronomical forcing;
- PALEOCEANOGRAPHYDE: 4946 Milankovitch theory;
- PALEOCEANOGRAPHY