New Mexico Geological Society Annual Spring Meeting — Abstracts

Middle Permian strata of the Northwest Shelf in the southern Guadalupe Mountains, New Mexico consist of thick shallow marine carbonates interbedded with thinner siliciclastic intervals of uncertain origin. Most previous studies have categorically assigned Capitan-equivalent (Upper Guadalupian) siliciclastics intervals of the Northwest Shelf to either of two general depositional environments-shallow marine; or eolian with few sedimentological observations to support their interpretations. A detailed sedimentologic study of the three siliciclastic intervals in the upper 15-30 m of the Yates Formation supported by reconnaissance observations from the underlying Seven Rivers and overlying Tansill formations has provided strong evidence for a shallow marine depositional environment for these shelf siliciclastic intervals, contrary to prevailing interpretation.

The three siliciclastic units of the upper Yates were examined in detail for sedimentologic and petrographic characteristics within the area extending 5 km from the Capitan shelf-edge and for 36 km parallel to the shelf-edge. These upper Yates intervals are continuously traceable across the study area and well beyond to the west and into the subsurface to the east. All three units thicken slightly basinward prior to pinching out abruptly between carbonate grainstones of the outer shelf within a few hundred meters of the Capitan massive at the shelf edge.

The lowermost unit averages 1/2 m in thickness and compositionally is a silty peloidal wackestone. The upper two intervals are subarkosic in composition and average 4 and 6 m in thickness (middle and upper units, respectively). All three intervals are in sharp, planar contact with the underlying carbonate unit. Locally the basal contact is planar erosional with obvious erosion of the substratum in excess of 1 m of relief as evidenced by truncated tepee structures. The upper contact of each siliciclastic interval is gradational to locally sharp.

Texturally the siliciclastics range from subangular silt (<64mm) in the silty wackestone to well rounded lower very fine sand (0.09-0.064mm) in the subarkose units. Rare isolated well rounded frosted medium (0.25-0.5mm) quartz sand grains characteristic of the yates in the subsurface are present in the subarkose units, and are particularly abundant in the area of the pinchout of each subarkose unit. In contrast a feature unique to the silty wackestone is the presence of trace amounts of glauconite. Sorting ranges from well to very well sorted. The
heavy mineral suite is dominated by a well rounded and abraded zircon-tourmaline-rutile assemblage.

Biotics are rare within either the silty wackestone facies or the subarkose facies and are found in greatest concentration and diversity in proximity of the Capitan Reef. Biotics range from fusulinids and dasycladacean algae to various molluscan and brachiopod fragments. Evidence of burrowing infauna is extremely rare though local subtle fabric mottling is apparent in the subarkose facies.

Sedimentologically the two lithofacies are distinct. The silty wackestone is characterized by development of three facies along a shelfward progression. Nearest the Capitan is developed a discontinuous wavy crypt-algal laminated facies. Locally developed within this facies are oscillation ripples containing fusulinids aligned in and parallel to the ripple troughs. Further shelfward this facies grades into a wavy laminated fenestral facies. This facies inturn grades farthest shelfward into a wavy lamined facies containing ostracods and calcispheres. The subarkose facies exhibit a shelfward diminution of scale, diversity and abundance of primary sedimentary structures from the reef proximal area. On the basis of sedimentary structure distribution three facies have been recognized in the shelfward progression: 1) small scale cross laminated facies; 2) ripple cross laminated to plane parallel facies; and 3) non-laminated to wavy laminated facies (farthest from the shelf edge). This facies progression is interpreted to be the product of shelfward diminution of hydraulic energy.

Moreover, across the area examinated none of these sandstones exhibit evidence of channeling or down-cutting into the subjacent carbonate unit. They exhibit no fining or coarsening upward sequences, no features interpretable as evidence of beach or tidal sedimentation, and no vertical repretitive sedimentation patterns. No stratification types were identified as unequivocal eolian featires (i.e. inversely graded translatent laminae, adhesion ripples, high index ripples, deflation surfaces or associated interdune pond facies). All primary sedimentary structures are interpretable as being of subaqueous origin. Furthermore the relative increase in abundance of the medium sand fraction in the subarkose units near the pinch out is thought due to winnowing in the zone of greater hydraulic energy.

Siciclastic units of the Seven Rivers and Tansill formations are sedimentologically analogous to these upper Yates intervals. All are herein interpreted as sheet sands deposited in a shallow marine shelf environment. The shelf hydraulic regime was of moderate to low energy which diminished with increasing distance from the shelf edge. The silty wackestone facies represents deposition in a shallow subtidal to supratidal (?) environment. The subarkose facies represents deposition in strictly a shallow subtidal environment of perhaps only a few meters depth. Input of mature likely multicycle siliciclastics to the aqueous shelf environment may well have been via eolian and/or fluvial processes. However transport and uniform distribution across the shelf was by a variety of shallow marine process operating on various periodicities. These fine grain siliciclastics were winnowed within the outershelf hydraulic environment and subsequently transported in suspension across the Capitan massive to be deposited in the deep waters of the Delaware Basin.