Gas Bubble and Expansion Crack Origin of Molar-Tooth"Calcite Structures in the Middle Proterozoic Belt Supergroup, Western Montana"

1998 ◽  
Vol Vol. 68 (1998), ◽  
Author(s):  
George Furniss, John F. Rittel, Don
Keyword(s):  
Molar Tooth ◽  
Western Montana ◽  
Gas Bubble ◽  
Crack Origin ◽  
Belt Supergroup ◽  
Middle Proterozoic

SHRIMP U-Pb geochronology of volcanic rocks, Belt Supergroup, western Montana: evidence for rapid deposition of sedimentary strata

2000 ◽  
Vol 37 (9) ◽  
pp. 1287-1300 ◽  
Author(s):  
Karl V Evans ◽  
John N Aleinikoff ◽  
John D Obradovich ◽  
C Mark Fanning
Keyword(s):  
Volcanic Rocks ◽  
Sediment Accumulation ◽  
Western Montana ◽  
Ion Microprobe ◽  
Stratigraphic Sequence ◽  
River Region ◽  
Northwestern Montana ◽  
Belt Supergroup ◽  
The Town ◽  
Middle Proterozoic

New sensitive high resolution ion microprobe (SHRIMP) U–Pb zircon analyses from two tuffs and a felsic flow in the middle and upper Belt Supergroup of northwestern Montana significantly refine the age of sedimentation for this very thick (15-20 km) Middle Proterozoic stratigraphic sequence. In ascending stratigraphic order, the results are (1) 1454 ± 9 Ma for a tuff in the upper part of the Helena Formation at Logan Pass, Glacier National Park; (2) 1443 ± 7 Ma for a regionally restricted porphyritic rhyolite to quartz latite flow of the Purcell Lava in the Yaak River region; and (3) 1401 ± 6 Ma for a tuff in the very thin transition zone between the Bonner Quartzite and Libby Formation, west of the town of Libby. Combining these ages with those previously published by other workers for ca. 1470-Ma sills in the lower Belt in Montana and Canada indicates that all but the uppermost Belt strata (about 1700 m) were deposited over a period of about 70 million years, considerably reducing the time span from longstanding estimates ranging from 250 to 600 million years. Calculated sediment accumulation rates between dated samples indicates rapid, but not unreasonable, values for early Belt strata, with decreasing rates through time. These ages also suggest the inadequacy of previously published paleomagnetic data to resolve Belt Supergroup chronology at an appropriate level of accuracy.

Petrology, geochemistry, and diabase-granophyre relations of a thick basaltic sill emplaced into wet sediments, western Montana

2000 ◽  
Vol 37 (8) ◽  
pp. 1109-1119 ◽  
Author(s):  
Michael A Poage ◽  
Donald W Hyndman ◽  
James W Sears
Keyword(s):  
Pore Water ◽  
Large Volume ◽  
Western Montana ◽  
Mantle Melting ◽  
Soft Sediment ◽  
Sediment Pore Water ◽  
High Iron ◽  
Sediment Deformation ◽  
Soft Sediment Deformation ◽  
Middle Proterozoic

The Plains Sill is a thick diabase-granophyre body that intruded the wet sediments of the Middle Proterozoic Prichard Formation of the Belt-Purcell Supergroup. The diabase is a high-iron tholeiite geochemically compatible with large-volume mantle melting in an intracratonic rift environment. Evidence of emplacement into wet sediments includes thick zones of homogenized granosediments adjacent to the sill, soft-sediment deformation at sill contacts, and sedimentary ovoid structures possibly formed by local fluidization of sediments. Utilizing sediment pore water and driven by heat from the sill, the diabase was metamorphosed during crystallization and cooling, leaving hornblende as the dominant mafic phase. Continued retrograde alteration resulted in overgrowths of secondary hornblende and variable alteration of plagioclase to epidote. A miarolitic granophyre layer, up to 150 m thick, caps the diabase and appears igneous in origin. Locally the granophyre is anomalously thick, perhaps reflecting updip migration of granophyric fluid where the Plains Sill cuts upsection through the Prichard Formation stratigraphy.

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