Journal
PALAEOGEOGRAPHY PALAEOCLIMATOLOGY PALAEOECOLOGY
Volume 528, Issue -, Pages 35-49Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/j.palaeo.2019.04.034
Keywords
Varve; Raleoenvironment; Astronomical time scale; Time series analysis; Composition of lamina; Duration of laminae
Funding
- National Basic Research Program of China (973 program) [2014CB239102]
- National Science and Technology Major Project [2016ZX05034002, 2016ZX05060004, 2016ZX05027001]
- Young Scientists Fund of the National Natural Science Foundation of China [41202086]
- National Natural Science Foundation of China [41802175]
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As a typical sedimentary structure in fine-grained rocks, laminae are widely distributed in shales and mudstones in sedimentary basins. The Shahejie Formation (42.2-45.4 Ma) formed during the Eocene of the Paleogene in the Dongying Depression of China is recognized as a typical area to study lacustrine shale. According to the composition of different lamina, four types of couplets are identified, including carbonate-clay couplets, carbonate-organic couplets, clay-organic couplets and carbonate-clay-organic triplets. All couplets are composed of light and dark layers. A combination of core images, microscopic observations, mineral compositions, geochemical data, carbon and oxygen isotopes, and strontium isotopes verifies that the laminae are primarily developed in a saline and anoxic, or even euxinic environment, with a high organic matter (OM) flux. Through astronomical cycle analysis of the natural gamma ray (GR) curve from sample site well NY1, the data suggest that the lacustrine shale laminae are formed annually with an average duration of 1.34 yr. The formation of laminae is affected by the season, which is closely related to the seasonal growth and death of algae. During the period of algae growth, a large number of light-colored carbonate laminae are deposited, whereas when the algae dies, organic matter accumulates at the bottom of lakes and forms dark organic-rich laminae. The interbedded and lenticular laminae, which are subdivided by layering characteristics, are primarily affected by diagenesis in postdepositional processes. Study of the laminae is helpful in understanding the formation process of lacustrine shale, and it provides invaluable sources of information for paleoclimate reconstruction.
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