Sequence stratigraphy of the upper jurassic evaporite-carbonate: Sequence at the western area of Wadi Al-Jawf-Marib basin, Yemen

Abstract

The Upper Jurassic evaporite-carbonate sequence (Sabatayn Formation and its equivalent Madbi Formation or Transition Beds) and its constituent systems tracts have been recognized at the western side of Wadi Al-Jawf-Marib intracratonic rift basin and its adjacent shelf areas. The Sabatayn Formation was subdivided, from the base to the top, into the Shabwa Salt Member, the Layadim Shale Member, the Ayad Gypsum Member and its equivalent M’qah Sandstone Member. The evaporite depositional systems of the lowstand facies (or systems) tract LST began in Middle Kimmeridgian times after the complete drawdown of the shelf area and basin isolation. The effect of ephemeral flooding events of the marine water and/or surface run-off water on the basinal salt pans resulted in the deposition of four or more parasequences (shallowing upward salinas of the Shabwa Salt Member).

When the rift was reconnected to the open ocean in Late Kimmeridgian times, the basinal LST and the formerly exposed carbonate sequence of the shelf area were flooded and an onlapping retrogradational transgressive facies tract (TST) represented by the lagoonal-sabkha Layadim Shale and the Ayad Gypsum members and their equivalent shoals, shallow subtidal to intertidal Madbi Formation were established. The continued relative sea-level rise followed by relative sea-level falls due to orogenic movement during Portlandian times resulted in the deposition of aggradational to progradational offlapping strata of early and late highstand facies tract (HST representing part of the Naifa Formation). On the shelf area, the early HST consists of a subtidal ammonitic limestone which changes westwards into a shallow subtidal-intertidal sandy rippled mudstone containing fossil fish remains and varicolored cross bedded sandy mudstone containing plant remains, while the late HST consists of supratidal to sabkha gypsiferous shale and sandstone with gypsum lenses. However, on the shelf slope, a brecciated ammonitic limestone interbedded with calcareous mudstone containing pelagic fauna represent the forced regressive wedge-systems tracts (FRWST).