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High-resolution organic record of Bridge Creek deposition, northwest New Mexico

Organic Geochemistry
Publication Date
DOI: 10.1016/0146-6380(94)90100-7
  • New Mexico
  • Cenomanian-Turonian Boundary
  • Sequence Stratigraphy
  • Bridge Creek
  • Transgressive Systems Tract
  • Homohopane Index
  • Anoxia
  • Th/U Ratio
  • Chemistry
  • Earth Science


Abstract Organic and inorganic geochemical data are presented and interpreted for a high-resolution sample set from the uppermost portion of a transgressive systems tract/condensed interval spanning the Cenomanian-Turonian boundary (at Red Wash, northwest New Mexico). Source potential (TOC, Rock-Eval yields), thermal maturity (VR), elemental (Ca, Th and U in whole rock) and molecular ( n-alkanes, steranes, terpanes) data are presented for a 64-sample, 12.4 m composite section of limestones, marlstones and shales of the Bridge Creek Member of the Greenhorn Formation. Vertical sampling resolutions of 1 spl/19 cm (spatial) and 1 spl/33 ka (temporal) are used to monitor the progress of the transgression from the uppermost Cenomanian up to the beginning of the highstand systems tract. The section is thermally-immature and maturity fluctuations are absent, indicating that organic parameters are a function of organic matter production and preservation (including diagenesis). The rhythmic lithologic fluctuation within the Bridge Creek is not reflected in the source potential, elemental or molecular data, which show either no obvious pattern (e.g. carbonate content, TOC) or a unidirectional trend toward development of anoxia upsection (e.g. hydrogen index, sterane distributions, homohopane index). The distribution of the triaromatic steroid hydrocarbons (C 20–21, 26–28) according to side-chain-length also varies significantly, despite the absence of maturity differences. All of the unidirectional trends in the data are interpreted as related to either productivity differences (e.g. sterane/hopane ratios, and changes in the relative concentration of αααR-C 28 and αααR-C 30 desmethylsteranes) or the development of anoxia in the lower water column (e.g. decreases in the diasterane/regular sterane ratio, increases in the homohopane index). Westward movement of the paleoshoreline during Bridge Creek time is also monitored by upsection-decreasing Th/U ratios at Red Wash.

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