Baker Pass, Jackson Co., Colorado, USA

Age: Cenozoic (5.333 - 56 Ma)

Description: Intermediate to felsic compositions

Lithology: Major:{mafic hypabyssal,felsic hypabyssal}

Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052. [133]

Age: Proterozoic (1600 - 2500 Ma)

Lithology: Orthogneiss/paragneiss

Reference: Chorlton, L.B. Generalized geology of the world: bedrock domains and major faults in GIS format: a small-scale world geology map with an extended geological attribute database. doi: 10.4095/223767. Geological Survey of Canada, Open File 5529. [154]

Age: Proterozoic (1600 - 2500 Ma)

Description: Conspicuously banded rock marked by alternating layers of contrasting composition; dark-gray to black, medium- to coarse-grained layers are rich in biotite, sillimanite, and magnetite; light- to medium-gray or bluish-gray, fine-grained layers contain more quartz and feldspar than biotite; white, coarse-grained layers consist of quartz and feldspar and minor biotite. Weathers to mottled reddish or yellowish brown. Layering is typically folded into moderate warps and tight flexures and isoclines. Protolith consisted of alternating beds of shale, siltstone, and sandstone. Regional metamorphism at about 1,700 Ma (Peterman and others, 1968, p. 2284) produced characteristic mineral assemblages that vary across the region depending on maximum local temperature and pressure. Northeast of the North Fork Big Thompson River, maximum temperature of metamorphism was as low as 525 degrees C (J.W. Drexler, oral commun., 1986), partial melting was not initiated (migmatite is absent), and the typical prograde assemblage consists of quartz, plagioclase, lepidoblastic muscovite, biotite, staurolite, andalusite, and iron oxides. Throughout most of map area, peak temperatures probably exceeded 700 degrees C (Nesse, 1977, 1984), anatectic melting was widespread (migmatite is ubiquitous), and typical mineral assemblage contains quartz, plagioclase, microcline, sillimanite, biotite, +/- garnet, +/- cordierite, hercynite, and iron oxides. Presence of garnet (rare) and (or) cordierite depends on local bulk composition; prograde cordierite appears to have formed by reactions of the type: biotite + sillimanite = cordierite + anatectic melt. Mineral changes between the lower and higher temperature regions are shown by the red dashed-line boundaries: sillimanite is present west of the S boundary, whereas staurolite and andalusite disappear near this boundary; microcline appears and lepidoblastic muscovite disappears at the K boundary; migmatite appears south and west of the M boundary; cordierite is present south and east of an unmapped, broad zone that extends from Eagle Rock (north of Estes Park) through Beaver Meadows, Mount Wuh, Ptarmigan Mountain, and southwestward to the east end of Granby Reservoir. GRI Source Map ID 141 (I-1973).
** Areas where numerous sills, dikes, or irregular bodies of Silver Plume Granite (Ysp) intrude biotite schist (Xs) [indicated by stippled regions on source paper map]. Areas that contain numerous conformable layers of pegmatite (YXp) [indicated by crossed regions on source paper map].

Lithology: Metamorphic

Reference: National Park Service Geologic Resources Evaluation program. Digital Geologic Map of Rocky Mountain National Park and Vicinity, Colorado. National Park Service. [51]