Colorado Headwaters Laramide Hydrogeologic Geologic Period Phase Stratigraphic Unit Unit Modern- Alluvium associated with present Quaternary Alluvial Aquifers Glaciation rivers Grouse Mountain Basalt Volcanics Neogene Extension Troublesome- Troublesome- North Park Formations North Park Aquifer Rabit Ears Volcanics Volcanics Transition White River White River Formation Aquifer Paleogene Coalmont Formation Coalmont Aquifer Laramide Middle Park Middle Park Formation Aquifer Windy Gap Volcanic Volcanics Formation Middle ParkMiddle Member Pierre confining Pierre Shale unit Cretaceous Interior Regional Cretaceous Seaway shale- Seaway dominated formations form multiple hydrogeologic units, most are confining units Colorado Piedmont Region Jurassic Multiple sedimentary units deposited Mesozoic in the stable continent interior may be Sandstones Triassic present that may form aquifers Permian Ancestral Rocky No strata Pennsylvanian Mountains Mississippian Devonian Paleozoic Silurian No strata Carbonates Ordovician Cambrian Crystalline rocks of igneous and Crystalline Precambrian Precambrian metamorphic origin in mountainous bedrock region Table 11b-02-08-01. Colorado Headwaters Laramide Basin stratigraphic chart. Colorado Headwaters Laramide Unit Thickness Hydrogeologic Geologic Period Phase Stratigraphic Unit Physical Characteristics Hydrologic Characteristics (ft) Unit Modern- Quaternary Alluvium associated with present rivers Alluvial Aquifers Glaciation Grouse Mountain Basalt Basalt flow caprock Volcanics Neogene Extension Troublesome- Unconfined aquifer where tuffaceous siltstone is absent, confined where Tuffaceous siltstone, fine grained sandstone, volcanic ash, conglomerate, Troublesome- North Park Formations 1,000-1,800 North Park siltstone is present; breccia, sandstone, conglomerate, and ash yield water volcanic fows and tuff breccia Aquifer to springs Interbeded tuff, tuff breccia and volcanic breccia interlayered with flows Rabit Ears Volcanics 0 - 1,500 Volcanics of intermediate composition Transition White River White River Formation Continental lakebed deposit Limited aquifer Aquifer Paleogene Large yields of potable water can be expected from poorly consolidated Conglomerate, sandstone, siltstone, and shale with some coal; poorly to Coalmont Formation 6,500 Coalmont Aquifer coarse-grained sandstone and conglomerate; will have limited well yield moderately consolidated. Predominately shale in central part of basin where shale predominates; water quality degrades with depth of aquifer Laramide Upper thick member of grit, sandstone, conglomerate and shale; lower Middle Park Middle Park Formation 2,500-5,000 member of volcanic material Aquifer Windy Gap Volcanic 0-700 Gray volcanic breccia, conglome te and andesitic flows Volcanics Formation ra Middle Middle Park Member Pierre confining Pierre Shale 200-4,500 Shale, siltstone with interbedded sandstone Sandstone layers might yield limited water Cretaceous Interior unit Seaway Regional Cretaceous Seaway shale-dominated formations form multiple hydrogeologic units, most are confining units Jurassic Colorado Mesozoic Multiple sedimentary units deposited in the stable continent interior may be present that may form aquifers Piedmont Region Triassic Sandstones Permian Ancestral Rocky No strata Pennsylvanian Mountains Mississippian Devonian Paleozoic Silurian No strata Carbonates Ordovician Cambrian Crystalline Precambrian Precambrian Crystalline rocks of igneous and metamorphic origin in mountainous region bedrock Table 11b-02-08-01. Colorado Headwaters Laramide Basin stratigraphic chart, detailed. Colorado Geological Survey ON-010 Colorado Groundwater Atlas..