Abstract

The Sonora Canyon centered in western Sutton County is one of several “Canyon Sands” (Virgilian-Wolfcampian) intervals that exist in the Val Verde Basin. Sonora Canyon sandstones formed in continental-slope and basin-floor depositional systems basinward of a northwest-trending shelf margin. The shelf-to-basin paleophysiography is defined by underlying Desmoinesian-Missourian carbonates. Well log analysis of the Sonora Canyon interval revealed an overall wedge-shaped geometry, thickening southwestward from 500 ft at the shelf margin to 2,000 ft where sandstone grades into basinal shale. Net-sandstone thickness reaches 1,000 feet in the middle of the wedge. During Sonora Canyon deposition, sandstone depocenters shifted northward and northeastward, onlapping the slope.

The Sonora Canyon comprises coalesced submarine-fan systems forming a slope apron. Deltaic sandstones occur locally along the shelf margin, but mudstone covers most of the outer shelf. On the upper slope, channel-fill sandstones are discontinuous at 0.5- to 1.5-mi well spacing and are enclosed in thick slope mudstones. Fan lobes, which include a proximal channelized part and a distal thin-bedded part, are best developed on the lower slope and adjacent basin floor. Individual lobes are 50 to 250 ft thick and about 5 mi in diameter, although coalescence obscures lateral lobe boundaries.

Turbidites are the most prominent sedimentary features identified in Sonora Canyon cores. Channel-fill sandstones are massive to normally graded (Bouma divisions A and B) and are associated with slump and debris-flow facies. Fan-lobe sandstones are composed of turbidites having more complete Bouma sequences. Thick-bedded turbidites having sandstone components 1 to 5 ft thick and mudstone components <0.5 ft thick occur primarily in proximal fan lobes, whereas distal lobes are dominated by thin-bedded turbidites having subequal sandstone and mudstone components <1 ft thick.

Sonora Canyon sandstones are mostly fine to very fine grained sublitharenites and litharenites. Chert, mudstone, sandstone, and low-rank metamorphic rock fragments are the predominant lithic grains. Major diagenetic events were (1) siderite and chlorite cementation, (2) mechanical compaction, (3) quartz cementation, (4) feldspar dissolution and illite and kaolinite precipitation and (5) ankerite cementation. Porosity is higher in samples with abundant siderite cement because early precipitation of siderite inhibited later mechanical compaction and quarts cementation.