Download e-book for iPad: Middle Proterozoic Belt Supergroup, Western Montana: Great by Don Winston, Robert J. Horodyski, James W. Whipple

By Don Winston, Robert J. Horodyski, James W. Whipple

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Published by way of the yankee Geophysical Union as a part of the Field journey Guidebooks Series.

The ideal want of the 3 leaders is this box journey be a discussion board for trade of rules between all contributors. the 3 people have differing viewpoints of the Belt basin, and every will current his personal principles. we are hoping the consequent dialogue will stimulate reaction from visitor individuals, whose mixed studies in some of these rocks most likely exceed ours. We motivate individuals to remark and to give a contribution to extra knowing those curious rocks.

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Read or Download Middle Proterozoic Belt Supergroup, Western Montana: Great Falls, Montana to Spokane, Washington, July 20-28, 1989 PDF

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Concerning the ProductPublished through the yank Geophysical Union as a part of the Geophysical Monograph sequence. content material:

Additional info for Middle Proterozoic Belt Supergroup, Western Montana: Great Falls, Montana to Spokane, Washington, July 20-28, 1989

Sample text

Except for shape, laminae in these stromatolites are similar to those occurring in the upper part of the underlying unit. These stromatolites commonly form stromatolitic bioherms, which range from 5-20 m in diameter and are separated by 1-2 m wide channels filled with thinly bedded finegrained limestone (probably eroded from stromatolitic mats) and silty and sandy dolomitic limestone. Stromatolite- debris beds of the type that occur in the underlying Lower Baicalia Unit are not present in this unit.

The early phases of attenuation and basin subsidence were accompanied by extensional growth faults, some of which may have involved thinned basement, particularly over and on the flanks of the thermal uplift. Grabens and half grabens formed in the western part of the basin during this extensional period and became depocenters for early Belt sediments that were intercalated locally with submarine volcanics and coeval sills and exhalative metal deposits. Basal, coarse-grained clastics were deposited along parts of the north, east, and south margins of the basin.

Low energy, not shallow water depth, probably is an important factor for the development of Conophyton (elsewhere Conophyton has been interpreted as a deeper water form). Alternatively, increased resistance of the stromatolitic laminae to breakup could have played on important role in the formation of Co nophyton, as peaked growth surfaces could resist breakup if they were stronger (perhaps through increased calcification). Motile filamentous microorganisms probably played an important role in the formation of conical laminae as well as the ridges in columns with lanceolate, sphenate, and stellate laminae in transverse sections.

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