A sequence stratigraphic analysis of the nearshore West Cameron and East Cameron blocks, central Gulf of Mexico, was developed for Lower Miocene age strata. Interpretation of each depositional sequence included delineation of paleoshelf/slope breaks, isopach, seismic facies, and paleogeography maps. Sequence development and sedimentation patterns were influenced by two prominent structural styles, growth faulting and salt/shale deformation. Two depositional sequences A and B, were identified and interpreted to consist of depositional environments ranging from submarine fan to coastal and shallow marine deltaics. Sequence A is the oldest Lower Miocene sequence and is divided into two depositional packages, a lowstand portion and a condensed interval. Because the paleo-shelf break was landward of our study area, there is an absence of well-developed shelfal facies in this sequence, and the condensed interval represents the distal transgressive and highstand portion of the sequence. The lowstand portion suggests a north to northwest source direction and has a maximum thickness of 2200 feet. The axis of maximum thickness, derived from isochron mapping, integrated with paleogeographic interpretation, indicates that the thickness or aggradation is related to mass-wasting processes and channel systems characteristic of submarine fan environments. The influence of pre-existing topography and basin configuration on sequence development and stratal architecture is evident where the axes of maximum thickness are separated by thin intervals of little to no deposition, related to a paleostructural high. Sequence B is better developed than Sequence A because of its location within the axis of the depocenter and can be subdivided into the following depositional environments: well-developed prograding submarine fan and slope deposits (associated primarily with falling sea-level) that are capped updip by an erosional surface associated with a marine transgression; an overall transgressive unit composed of localized regressive deltaic deposits; and aggradational fluvial/deltaic and shallow marine sediments associated with a continued rise in sea-level. Sequence B has a maximum thickness of 3200 feet and, like Sequence A, this aggradation within the basin is attributed to mass-wasting and channel development within a slope envronment. The variation in location of thickness trends, combined with evidence of erosional channels from different source directions, indicates the migration of sediment supply through time from a more westerly to a more northerly direction.
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.
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