DEPOSITIONAL AND DIAGENETIC ENVIRONMENT OF THE BRUSHY BASIN MEMBER

Sediments of the Brushy Basin Member below the Jackpile sandstone were deposited in an extensive hydrologically closed basin occupied in part by a playa surrounded by mudflats. These in turn were surrounded by a low-gradient alluvial plain which was traversed by braided and anastomosing perennial or ephemeral streams. Zeolitic mudstone, calcareous mudstone, and bentonitic mudstone interbedded with isolated shoestring sandstones are the major sediment types in each of these respective environments.

Surface runoff and shallow ground water were the major source of solutes to the playa brine. Sediments of the Brushy Basin Member originated in a terrain of felsic igneous and clastic metasedimentary rocks. Runoff from such a terrain will consist of silica rich, soft, Ca-K-Na-HCO3 waters11. Evaporitive concentration and the resulting fractionation of waters of this type yield a sodium-bicarbonate brine11. Extensive silicic volcanism during Brushy Basin time supplied abundant volcanic ash to the depositional system. Undisturbed altered ashfall tuffs and reworked ash mixed with the feldspathic sediment comprise the bulk of the mudstones. Dissolution of this ash in the alkaline waters yielded additional alkalies and silica to the playa brine.

Figure 2

Stratigraphic column of the upper part of the Morrison Formation in the southern San Juan basin.

Ash altered differently in the three environments, reflecting the pH and solute concentration of the pore fluids. Ash was altered to montmorillonite in alluvial-plain sediments where the pore fluids were relatively dilute. Calcite and montmorillonite replaced ash in mudflats at the playa margin where fresh ground water and surface runoff mixed with the carbonate-rich playa brine. Ash in the playa sediments was altered to montmorillonite, calcite, the zeolites heulandite-clinoptilolite, and analcime. The distribution of these authigenic minerals attest to the pH and salinity gradients which existed within the basin during and shortly after deposition. At the close of Brushy Basin time, the Jackpile sandstone prograded over the mudflat and playa sediments. The thickest portions were localized in growing Jurassic synclines which trend northeast through the Laguna District21. In general, stream directions and consequently sediment thickness were largely controlled by tectonic activity during Morrison time8.