--> ABSTRACT: Sequence Stratigraphy of the Upper Ojinaga and San Carlos Formations, Late Santonian to Campanian, Sierra Vieja Region of Western Texas, by Cheryl L. Metz; #91020 (1995).

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Sequence Stratigraphy of the Upper Ojinaga and San Carlos Formations, Late Santonian to Campanian, Sierra Vieja Region of Western Texas

Cheryl L. Metz

The upper Ojinaga and San Carlos Formations in the Sierra Vieja region, Presidio County, Texas record the progradation of terrigenous classic sediments eastward over the Diablo Platform during Late Cretaceous time. Changes in the relative sea level and strandline position over the platform are documented by eight parasequence sets.

Two sequences in the upper Ojinaga Formation are recorded by a shelly condensed section containing a possible methane-seep mound (Oj 1) and by an overlying interval of basinal marine shales which grade upward into silty mudstones and hummocky-cross stratified (HMS) sandstones (Oj 2). The upper sequence is terminated by an erosional surface and thin lag deposit of shell and bored lithoclasts, which forms the boundary between the Ojinaga/San Carlos Formations.

The basal San Carlos sequence (SC 1) comprises several well developed parasequences of interbedded shale, silty mudstones and upward thickening hummocky-cross stratified sandstones. These parasequences represent progressive shoaling along a storm dominated shelf during the Late Santonian-Early Campanian. The basal San Carlos sequence can be terminated by one of two overlying sequences. The lower sequence is bounded by a shell and bored lithoclast lag overlain by HMS sandstones and thin silty mudstones (SC 2). The upper sequence (SC 3), which can cut down into the top of SC 1 parasequence set, consists of alternating marine, coastal and deltaic facies deposited along a fluctuating coast line during the Middle Campanian.

During a major transgression at the end of the Middle Campanian, the Diablo Platform was again inundated resulting in the formation of condensed section upon which marine shales were deposited (SC 4). A rapid drop of sea level in the Late Campanian deposited fluvial-deltaic and alluvial plain facies (SC 5) directly over the basinal marine shales. Changes in the geometry and style of fluvial sedimentation in these overlying nonmarine strata of the Upper San Carlos Formation may be related to a transgression that shifts base level within the area (SC 6).

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995