Abstract

The Chuska sandstone as here restricted forms the cap rock of the Chuska Mountains, which extend 60 miles along the Arizona-New Mexico State line north of Gallup, New Mexico. Its maximum preserved thickness is about 1750 feet. It is unfossiliferous, cross-bedded throughout, and contains no shale or conglomerate interbeds; there are several thin ash beds. The cross-bedding dips consistently in the northern quadrants at angles around 25°. Alternating strong to weak chalcedonic and opaline cement in thick layers throughout the formation provides a topography of alternating resistant ledges and gentle slopes.

Rosiwal analyses of four samples averaged 74 per cent quartz and 23 per cent feldspar. Sixteen samples have an average median diameter of 0.21 mm and are well sorted and slightly skewed. These values are compared with those of eolian sands described in the literature, and an evaluation is made of the use of particle-size parameters as criteria for postulating depositional environment.

The average intercept sphericity for 8 samples is 0.78; the average roundness for 15 samples is 0.51. Nine samples showed that an average of 29 per cent of the grains had frosted or pitted surfaces. Shape, roundness, and frosting are probably inherited from source sediments.

The nature of the cross-bedding and the uniformity of texture imply accumulation of dune sand by winds from the south-southwest. Available median diameters, sphericities, roundness, and frosting were plotted against distance in the presumed direction of transport. No firm trends are evident in the 38 miles between outside samples.

The restricted Chuska sandstone and the underlying fluvial Deza formation, together forming Gregory's Chuska sandstone, truncate the Mesaverde group (Upper Cretaceous) and are unconformably overlain by middle or upper Pliocene volcanics. The suggested Miocene (?) age is based on physiographic data.

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