New Mexico Geological Society Annual Spring Meeting — Abstracts

The interbedded sandstone and carbonate strata of the Permian upper Yates and lower Tansill Formations on the Northwest Shelf of the Delaware Basin New Mexico were studied to determine their sedimentologic and temporal relationships. The evidence suggests local minor tectonic, as opposed to eustatic, control of carbonate sedimentation and of the sandstone-carbonate alternations. Along the most basinward 5km of the Northwest Shelf the Yates-Tansill study interval consists of three sandstone-carbonate alternations with a composite thickness of up to 40m. The carbonates vary from skeletal grainstones to nonskeletal wackestones and range up to 12m thickness each. The
sandstones are very well sorted, very fine grain siliciclastic units from 1/2-8m thickness, and range from silty peloidal wackestones to subarkose arenites. The yates-Tansill interval grades laterally basinward into the Capitan Reef facies at the shelf edge.

Within each of the carbonate intervals a number of peritidal facies have been recognized which collectively exhibit upward shoaling characteristics. The upward shoaling sequence is typically 1-2m thick each, and is hemi-cyclic. Several of these sequences may exist in vertical succession in each carbonate unit examined. Commonly a hemi-cyclic sequence is capped by a ragged erosion surface which is overlain by another similar hemi-cycle. The stacking of these truncated upward shoaling sequences is interpreted to be the product of peritidal sedimentation to depositional-fill level in response to episodic relative shelf subsidence of comparable magnitude. The upper few ems of many hemi-cycles evidence peritidal diagenesis interpretable in some cases as evidence of emergence. In two instances the diagenesis is analogous to incipient pedogenesis implying extended subaerial exposure. In contrast, the contemporaneous shelf carbonate facies further basinward and further shelfward do not exhibit upward shoaling sedimentation, erosion, or comparable diagenesis. The area of upward shoaling peritidal facies is interpreted to represent a paleotopograhic high or crest of the geomorphic shelf marginal mound.

The relationship of the marginal mound to sandstone deposition has long been enigmatic. The interbedded sandstones are seemingly structureless sheet sands that are continuous across the study area including the area of shelf crest facies in the subjacent carbonate interval. Within a few hundred meters of the shelf edge each sandstone interfingers with and abruptly pinches out into the reef-proximal shelf facies. In general, all three sandstone intervals are in sharp but conformable contact with the underlying carbonate units except across the area of shelf crest facies where the basal contact is clearly erosional and the carbonate substratum locally exhibits evidence of incipient pedogenesis. In marked contrast, the upper contact of each sandstone with the overlying carbonate unit is typically of a more gradational nature.

The sandstones exhibit no evidence of down-cutting, or coarsening or fining upward sequences. Primary sedimentary structures are rarely visible, where present they are small scale (<20cm) and discontinuous. Their apparent absence is due to high textural maturity (very fine grain size <0.1mm, and high degree of sorting) and/or thorough reworking by infauna. The overall distribution of sedimentary structures indicates subaqueous deposition within an hydraulic regime of shelfward diminishing energy. Contrary to most previous interpretations,
no evidence of eolian stratification was found to support an eolian depositional environment for the shelf sands.

Shelf sandstone transport processes remain enigmatic but are dominantly basinward directed and likely of marine origin. Siliciclastic input to the marine shelf environment is believed to have been slow but continuous along the strandline. The effectiveness of marine transport of siliciclastics across the shelf was governed largely by minor episodic relative shelf subsidence which periodically increased or maintained water depth favorable for subaqueous transport of shelf siliciclastics. Apparent unrestricted marine transport of quartz sands across the
most basinward 5km of the shelf requires the absence or lack of influence of any hydrographic barrier such as a marginal mound during sandstone depositional episodes. This condition was repeatedly met by a combination of erosional truncation of the shelf crest facies of the marginal mound and subsequent increased shelf water depths due to relative subsidence of a few meters, at which time siliciclastics were transported basinward by a variety of marine shelf processes.

The gradation of sandstone upward into upward shoaling hemicyclic peritidal carbonates along the crest of the shelf marginal mound is interpreted to be a dual lithology depositional couplet. This couplet is unique to the area of the shelf crest and consists of a lower sandstone member grading upward to upward shoaling peritidal carbonates which became episodically emergent locally. In this "all wet" shelf interpretation, the sandstones represent the subtidal sedimentation and the carbonates represent the intertidal-supratidal sedimentation.

The interpretation of minor (1-2m) epiisodic relative shelf subsidence controlling the alternation of shelf carbonate and sandstone deposition is an alternative to the long professed dogma of dominantly eustatic control of Northwest Shelf depositional sequences. This interpretation is preferable to previous interpretations in that it is documentable on the scale of an outcrop, it is consistant with the observed sedimentologic and stratigraphic relationships of the Yates-Tansill sequence, and it may be applicable to comparable stratigraphic sequences elsewhere inn the Permian Basin.