Sequence Stratigraphic Analysis of Upper Turonian and Coniacian Strata in the San Juan Basin, New Mexico, U.S.A.
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This paper derives the absolute sea level history for the Late Turonian and Coniacian strata in the San Juan Basin of New Mexico. The sea level history is deduced from (a) the degree of aggradation versus progradation of coastal sandbodies, (b) paleoenvironmental interpretation of the successive depositional systems tracts, (c) location and extent of basin-margin subaerial unconformities, (d) location and extent of marine condensed sections and their associated hiatuses and (e) the presence of distinctive transgressive surfaces. Dates are assigned to this sea level history based on well-established ammonite and inoceramid zonations for the western interior and the absolute time scale of Haq and co-workers.
The analysis applies sequence stratigraphic concepts and principles to the interpretation of a set of shallow marine, clastic-dominated depositional systems and their bounding surfaces. The following systems tracts are clearly discriminated by the paleoenvironmental analysis: highstand (early and late), lowstand (early and late), and transgressive. In addition, the subaerial unconformity (sequence boundary) and the submarine condensed section with its associated hiatus are readily differentiated based on the appearance of the surfaces themselves and the associated lithofacies characteristics.
The inferred history of deposition and sea level change is as follows. A sequence boundary is located at the base of the Late Turonian Juana Lopez Member of the Mancos Shale (90 Ma). This probably corresponds to a period of rapid eustatic fall. The subsequent episode of maximum flooding occurred about 0.5 Ma later and was followed by aggradation of Gallup Sandstone Tongues F, E, and D in the early highstand systems tract. Gallup Sandstone Tongue C prograded during late highstand and early lowstand. A depositional sequence boundary forms an unconformity on top of the landward part of the Gallup C-Tongue and a conformity at its seaward base. This boundary is dated as Early Coniacian (89 Ma). A slow transgression occurred during most of the Coniacian attaining maximum landward flooding of the overlying Mulatto Shale Tongue at the end of Coniacian age (88 Ma).
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