Sequence Stratigraphic Interpretation of Coniacian Strata in the San Juan Basin, New Mexico

Abstract

Detailed measured sections, subsurface data and regional observations along the southern and western margins of New Mexico’s San Juan Basin provide a precise data base for testing proposed sequence stratigraphic models which describe the architecture of depositional sequences. It is concluded that Coniacian strata in this basin represent lithosomes deposited during relative sea level lowstand and subsequent transgressions. The base of the Torrivio Member of the Gallup Sandstone records the landward, unconformable Coniacian sequence boundary. The uppermost tongues (C-A) of the Gallup Sandstone and the associated upper member of the Lower Mancos Shale represent the lowstand wedge. The base of this wedge is the seaward, mostly conformable, continuation of the basal Torrivio sequence boundary. In distal shelf settings, beyond the influence of the Gallup highstand deposits (highstand systems tract), the sequence boundary merges with the condensed section of the next underlying sequence—the Juana Lopez Member of the Mancos Shale. The Tocito Sandstone represents a set of linear shelf sand ridges which formed during transgressions associated with parasequence boundaries within this regressive lowstand wedge. Ridge-building also continued into the overlying transgressive systems tract. Dominant ridge-building currents were along-shelf tidal flows, partly modified by geostrophic storm flows. As articulated in sequence stratigraphy, analysis of the genetic linkage between contemporaneous depositional systems and the associated relative sea level stage is a powerful tool in stratigraphic and sedimentological analysis. The many conflicting interpretations already published on the Gallup-Tocito stratigraphy partly owe their origin to a lack of appreciation for the direct connection between styles of sedimentation and patterns of relative change in sea level.

“A la recherche du temps perdu”: On geological condensation, with examples from the Jurassic Subbetic Plateau in Southeastern Spain

“…Winnowing is very common on clastic shelves; Middle jurassic sequences from the northern border of the Tethyan ocean contain numerous condensed deposits produced by burial-exhumation-reworking cycles (compilations in DIETL 1977, URLICHS 1977, FURSICH 1979, HALLAM 1988. Shell concentrations on clastic shelves and bonebeds on carbonate platforms are often clearly related to episodes of rising sea-level; in most cases, they are the result of a combination of filtering and winnowing (KIDWELL 1985(KIDWELL , 1986(KIDWELL ,1987FURSICH & KAUFFMAN 1984;NUMMEDAL et al 1986;NUMMEDAL & SWIFT 1987;HAGDORN & REIF 1988, FURSICH et al 1991. Scouring requires previous lithification and is mostly the result of current activity (BROMLEY 1975, MARTIRE 1992, WILSON & PALM ER, 1992.…”