Stratigraphy, sediment dispersal and paleogeography of the lower Eocene San Jose Formation, San Juan Basin, New Mexico
Larry N. Smith
The fluvial lower Eocene San Jose Formation represents the last preserved period of deposition in the Laramide San Juan Basin. The basal Cuba Mesa Member of the formation is a nearly basinwide, coarsegrained sheet sandstone composed of multilateral and multistoried low-sinuosity fluvial channels. The Cuba Mesa is locally thickened where sheet sandstones are vertically amalgamated. The Cuba Mesa thins by intertonguing with mudrock and pinches out along the basin axis where the San Jose Formation conformably overlies the Paleocene Nacimiento and Animas Formations in northern New Mexico and Colorado. The Cuba Mesa Member unconformably overlies Paleocene and Mesozoic strata toward margins of the basin, with as much as 90° of angular discordance. Floodplain mudrock and disconnected sheet sandstones of the Regina Member were deposited laterally adjacent to and above the Cuba Mesa Member. Mudrock of the Regina Member also intertongues with the lobate, sandstone-dominated Ditch Canyon Member (named here) and Llaves Member. The Ditch Canyon Member represents southeast-directed fluvial deposition in the northwestern San Juan Basin from the Four Corners platform and southwestern San Juan uplift. The Llaves Member was deposited on the eastern side of the basin by west-southwest-flowing streams and is sedimentologically and stratigraphically similar to, but not continuous with, the Ditch Canyon Member. The Llaves Member is overlain by the siltstonedominated Tapicitos Member. The Tapicitos was derived from uplifts east of the basin. Grain size in the Regina Member decreases toward the San Juan and Nacimiento uplifts, due to minimal erosion of coarse-grained sediment from these active, reverse-faulted monoclines. Small drainage basins along these active mountain fronts eroded mostly fine-grained sediment from Phanerozoic strata. Coarse-grained sediment of the Ditch Canyon and Llaves Members was deposited by fluvial systems that emanated from different paleo-drainage basins within structural reentrants between basin-marginal uplifts. The Cuba Mesa Member was deposited during late Paleocene through early Eocene subsidence in the center of the San Juan Basin with concurrent erosion and possibly slow sedimentation nearer basin margins. Synsedimentary angular unconformitics within the Regina Member show that the Regina, Llaves and Ditch Canyon Members were deposited during episodic monoclinal folding near the Nacimiento fault.
- Smith, Larry N., 1992, Stratigraphy, sediment dispersal and paleogeography of the lower Eocene San Jose Formation, San Juan Basin, New Mexico, in: San Juan Basin IV, Lucas, Spencer, G.; Kues, Barry S.; Williamson, Thomas E.; Hunt, Adrian P., New Mexico Geological Society, Guidebook, 43rd Field Conference, pp. 297-309.