Late Jurassic Paleohydrology of San Juan Basin: Simulated Effects of Variable Transmissivity and Lake Levels: ABSTRACT

Abstract

A three-dimensional computer model of ground-water flow in the San Juan basin at the end of Morrison Formation time shows that variations in transmissivity and in the level of paleo-Lake T'oo'dichi would have significantly altered the direction of ground-water flow compared to that of surface flow. The model consists of four layers: the (1) Brushy Basin, (2) Westwater Canyon, (3) Salt Wash and Recapture Members of the Morrison, and (4) the San Rafael Group. Paleotopographic slopes were determined from alteration facies thought to indicate variations in extent and depth of the paleoplaya lake. Thicknesses of sandstones and mudstones from approximately 2,000 drill holes were used to estimate transmissivities and conductivities. Simulated results for the lowest paleolake levels and for constant fluid density show that ground water generally would have flowed from highlands on the west, south, and east northward to discharge areas in the lowest part of the basin. Low-permeability lacustrine sediments of the Brushy Basin Shale Member, northward increase in mud/sand ratio in fluvial sediments, east and southeast-trending wedges of thicker fluvial sands, and high lake levels would have partly redirected ground-water flow eastward and tended to cause upwelling and discharge in the southeastern part of the basin andmore » along the paleoshoreline. Downward fluid flow in the basin interior would have required higher fluid density in the center of the basin than at the margins.« less