Slope Discordance Cycles--Key to Erecting a Stratigraphic Hierarchy in Slope Systems: Middle Permian Brushy Canyon Formation, Guadalupe Mountains, TX
Mark D. Sonnenfeld and Michael H. Gardner
Slump scars are commonly observed among slope facies, yet are rarely placed in a cyclic context because slope facies successions are relatively poorly organized. Deep-water strata of the Brushy Canyon represent a 3rd-order lowstand wedge consisting of three seismically resolvable high-frequency sequences, each containing up to six slope-discordance cycles. A hierarchy of cross-cutting relationships establish the most laterally continuous discordance surfaces, whereas analysis of facies bounding major discordances constrain their timing of formation. Exceptional 3-d exposures not only define the spatial and temporal distribution of slope discordances, but also document their importance as major topographic lows (1-3 km wide, up to 45 m deep) partly infilled by multistory c annel-levee sandstone bodies.
Although slope facies are poorly organized, we recognize the following facies in order of decreasing inferred energy of deposition: (1) debris-flows; (2) calcareous clast-rich sandstones to debris flows; (3) high-density sediment gravity flows; (4) thin-bedded, ripple laminated turbidite sandstones; (5) interlaminated sandstones and siltstones; and (6) laminated organic- rich siltstones. The latter two facies record suspension sedimentation dissected by numerous small-scale slope discordances (<10 m wide); however, the major slope discordances are associated with, and often bracketed by the lowest energy, most organic-rich siltstones. Major slope discordances are often overlain by intermittent calcareous sandstone, sandstone, and siltstone facies prior to infill as preferred sites f channel-levee sandstone deposition. We interpret strata on both sides of the major discordances to represent deposition during to just after maximal relative sea-level rise, supporting timing of large-scale slope failure.
Whereas many deepwater models relate slope failure and sandstone transport to either maximum relative rise or fall of sea-level, we relate slope failure and the formation of important topographic lows in slope settings to times of high relative sea-level, with subsequent sand flux peaking at maximum relative sea-level fall. Multiple factors probably contribute to the observed timing of major slope failure such as: 1) increased pore-water pressures due to upslope loading by vertically stacked deposits and/or greater water column height; 2) the capability of organic- rich siltstones to behave as preferred glide planes and enhance slope instability; and 3) condensed slope sedimentation causing multiple slope failure events to coalesce, producing a compound discordance s rface. Slope-discordance-bounded cycles enable interpretation of basinward- and shelfward-stepping stacking patterns, thereby providing a systematic framework for understanding changes in facies arrangements among slope strata.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California