Other
Scientific paper
Feb 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996gecoa..60..667h&link_type=abstract
Geochimica et Cosmochimica Acta, Volume 60, Issue 4, p. 667-677.
Other
7
Scientific paper
Carbonates from the ~1100 1450 Ma old Proterozoic Belt Supergroup were collected from stratigraphic sections throughout Montana and Idaho, USA. The sampled sequences, in ascending stratigraphic order, include the Newland, Altyn, Spokane/Greyson transition, Empire, Wallace, Helena, Siyeh, Snowslip, Shepard, and Libby formations. An increase in the degree of postdepositional alteration of Belt limestones is reflected in a diminution of Sr and Mg contents, an increase in Mn, and depletion in 13C and 18O. Two diagenetic trends can be resolved for the limestones. One, affecting the presumed originally aragonite-rich sediments, includes carbonates from the Lower Belt Newland Formation. In contrast, the Middle Belt Carbonate (Wallace, Helena, Siyeh formations) may have been originally of high-Mg calcitic mineralogy. Projection of the alteration trends for the Lower and Middle Belt limestones suggest ~21‰ SMOW and ~ +2.5‰ to +1.0‰ PDB as the best preserved values for the δ18O and δ13C of seawater, respectively; both comparable to results from other Mesoproterozoic carbonate sequences. The oxygen isotope data for limestones show a regional westward depletion of ~8‰ in 18O, possibly reflecting a higher temperature of postdepositional alteration in the western Belt basin. This depletion in 18O is accompanied by a comparable decrease in δ13C values, most likely because a higher proportion of carbon was incorporated from CO2 generated by thermal cracking of hydrocarbons at depth. Dolostones in the Belt basin are dominantly micritic, with good preservation of depositional textures. Chemically and isotopically, their alteration trends mimic those of limestones, leading to comparable projected “best” values for δ13C. 87Sr/86Sr values of Belt carbonates range between 0.70484 and 0.74991. Progressive diagenesis, as indexed by decreasing concentrations of Sr and depletions in 18O and 13C, results in an increase in 87Sr/ 86Sr values. The least radiogenic measurement, from a Lower Belt Newland limestone sample, appears to fit reasonably well into the general trend for Proterozoic seawater, as discussed in Mirota and Veizer (1994). This and the consistency of δ18O and δ13C in limestones with other coeval sequences suggests that the (Lower and Middle) Belt carbonate sections are marine, deposited in an environment that was not continuously separated from the open ocean.
Hall Susan M.
Veizer Ján
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