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Sequence stratigraphy of the Oxfordian to Tithonian sediments (Baisakhi Formation) in the Jaisalmer Basin

Pandey D. K., Pooniya D.

Volumina Jurassica

2015

Vol. 13, no. 1

65--76

The Oxfordian to Tithonian sediments deposited along the southern Tethyan margin exhibit very diverse sedimentary facies. The Jaisalmer Basin, situated along the northwestern margin of the Indian peninsula, is a good example of those preserving these sediments (Baisakhi Formation). The scattered nature of the outcrops, due to peneplanation and desert sand cover, and the diversity in sedimentary facies, have led earlier workers to employ a confusing lithostratigraphy and lithostratigraphic correlations. New records of ammonites, more detailed facies description and the application of the sequence stratigraphic method helped to sort out the stratigraphical problems of these isolated outcrops. The complexity of the Oxfordian-Kimmeridgian-Tithonian sediments present in the Jaisalmer Basin, as evidenced by ammonite finds can be traced along an onshore – offshore transect. Within the Oxfordian-Tithonian strata of the Baisakhi Formation, Jaisalmer Basin, 35 parasequences grouped into three depositional sequences (TST-HST/FSST) have been recognized. In at least two cases, the maximum flooding surfaces (MFS) are superimposed on the transgressive surfaces within very condensed sections. The parasequences belong to a continuous succession representing shoreface (deepest part of the basin) to foreshore (shallowest part of the basin). During deposition of the Tithonian succession the southern part of the basin witnessed shallow, nearshore, or foreshore to fluvial depositional environments and, consequently, contains several depositional gaps and less distinct parasequences. The chronological order of the parasequences has been established on the basis of ammonites: Oxfordian – Perisphinctes (Dichotomosphinctes) sp. and P. (Dichotomoceras) sp.; Kimmeridgian – Torquatisphinctes alterniplicatus (Waagen) and Pachysphinctes aff. major Spath; Tithonian – Aulacosphinctoides sp., Virgatosphinctinae and other zonal ammonites recorded by earlier workers. The most logical facies correlation in conformity with Walther’s law was also used. Unfortunately, no marker beds have been recognized; however, the record of at least two brackish-water phases helped in the understanding of depositional settings along the onshore-offshore transects during these time intervals. In general, the Kimmeridgian-Tithonian part of Baisakhi Formation exhibits a general coarsening-upward trend and a change from lower shoreface zone to fluvial environments. The three sequence cycles interpreted within the Oxfordian-Tithonian sediments in the Jaisalmer Basin correspond to second-order sequence cycles.