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Abstract

The carbonate ramp to rimmed shelf transition is a well-known depositional pattern in carbonate platforms, but the nature of the stepwise evolution, the evolution of facies and facies tract patterns from sequence to sequence, and the intrinsic and extrinsic controls that drive the transition are less well known. The Guadalupian carbonate platform system in the southwestern US provides an example of a ramp-to-rim transition in a carbonate to mixed siliciclastic-carbonate succession during a second-order progradational supersequence. The sequence framework of the Guadalupian section includes 30 high-frequency sequences deposited over an 11 Myr time span. Compositing data from exposures across a 50 km strike width and 30 km dip width area provides the control needed to document this transition.

Early Guadalupian San Andres sequences exemplify low-angle (<2°) foreshore-shoreface-offshore profiles (ramps) with 60 to 80 m of topset to toeset relief and as much as10 km of slope width (distance from shelf inflection to slope inflection point). Extensive lower slope-basin hemipelagic mudstones of the Cutoff Formation were shed from this active mud factory, and this balance of accumulation rates in the platform and slope maintained a low-angle profile. A distinct lack of sedimentgravity-flow features is noted from this profile. After the protracted Brushy Canyon lowstand event, upper San Andres shelf carbonates show cessation of low-angle 2 to 4° ramplike profiles, where active carbonate factories on the shelf produced thick margins that tapered downdip into less-developed lower-slope hemipelagic mudstone aprons. Dilute toe-of-slope turbidites and upper-slope fusulinid banks add heterogeneity to this slope profile.

Uppermost San Andres and Grayburg sequences mark the beginning of a forced regressive mixed siliciclastic-carbonate pattern in which siliciclastics constitute 30 to 50% of the platform and slope. Uppermost San Andres clinoforms prograded between 5 and 10 km, range in slope angle from 4to15°, and have 100mof topset to toeset relief. A combination of siliciclastic and mixed carbonate-siliciclastic turbidites accumulated on the lower slope, and the upper slope consists of fusulinid-rich packstones with dips up to 20°. No debris deposits developed in the uppermost San Andres sequence. Rare patch reefs occupied shelf-margin promontories between reentrants that were dominated by siliciclastic bypass. The reduced (<5 km) carbonate factory width of the uppermost San Andres results in absence of a hemipelagic mudstone lower-slope apron, which accentuates oversteepening of the slope. The lower Grayburg-upper Cherry Canyon sequence can be considered a lowstand wedge complex within the longer-term succession. This Grayburg-Cherry Canyon interval prograded 5 km and is characterized by 5-to 25°-dipping mixed siliciclastic-carbonate clinoforms, 65 m of topset to toeset relief, and minor debris flows. As for the G9 high-frequency sequence, there is a narrow shelf carbonate factory (<5 km) and absence of a hemipelagic mudstone lower-slope apron.

Progradational and aggradational stacking of the middle Grayburg through lower Queen Formations most likely contains the record of the first reef-rimmed platforms with massive bedding developed near the shelf-slope break. A180-m-relief collapse scar at the margin as exposed at Bush Mountain on the Western Escarpment removed critical evidence of potential reef-margin development for the middle Grayburg-lower Queen sequences. Following this major collapse, the upper Queen (shelf)-Goat Seep (reef)-South Wells (basin) section represents the first preserved reef-rimmed platform margin. Clinoformheights of200m, a massive reefal margin-upper slope, and a debris-dominated foreslope with dips at angle of repose signal the final transitional stage into a Capitan-like system. Remaining Seven Rivers-Yates-Tansill-Capitan-Bell Canyon sequences continued to build relief to 500 to 600 m as they prograded an additional 5 km basinward. Slope complexes within the Capitan-equivalent section consist of compensationally stacked ponded and backfilling geometries separated by through-going collapse events.

Three primary but interrelated drivers are visualized for the ramp-rim transition recorded in Guadalupian strata of the Guadalupe Mountains outcrops. First, initial siliciclastic influx and concomitant reduced export of fine-grained carbonates to the slope during uppermost San Andres deposition led to basin starvation, margin steepening, and an increase in sediment bypass and early stage margin colonization. Second, middle Guadalupian Grayburg aggradational stacking of shelf margins drove progressive oversteepening and ultimately led to large-scale collapse of the margin, at least locally. This headwall became the site of the first preserved reefal margin in the Guadalupes. Finally, a marked increase in shelf accommodation space was created by flexural subsidence of Leonardian shelf strata over the buried Late Leonardian shelf margin. In other parts of the Delaware Basin, the steep Leonardian margin break was the single dominant control on the development of Guadalupian margins.

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