POSTDEPOSITIONAL CHEMICAL ALTERATION OF OUACHITA SHALES

Authors
Citation
Sj. Sutton et Ls. Land, POSTDEPOSITIONAL CHEMICAL ALTERATION OF OUACHITA SHALES, Geological Society of America bulletin, 108(8), 1996, pp. 978-991
Citations number
43
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Geosciences, Interdisciplinary
ISSN journal
0016-7606
Volume
108
Issue
8
Year of publication
1996
Pages
978 - 991
Database
ISI
SICI code
0016-7606(1996)108:8<978:PCAOOS>2.0.ZU;2-Y
Abstract
Shales belonging to the Carboniferous flysch exposed in the Ouachita M ountains of west-central Arkansas and southeastern Oklahoma have under gone postdepositional alteration that has significantly affected their whole-rock chemistry. Alteration is particularly pronounced in the no rtheastern Ouachitas and along the Benton-Broken Bow uplift, Altered r ocks have lost Ca, Mg, K, Na, Fe, and Si relative to the ''conservativ e'' elements Al, Ti, Zr, Cr, and Ni. Some evidence exists for slight l oss of Al and Ti relative to Cr, Ni, and Zr. Pyrophyllite- and chlorit oid-bearing samples are enriched in O-18 relative to less altered samp les, and this enrichment is not due to changes in the delta(18)O of qu artz. Mineralogical and textural evidence support a postdepositional a lteration model rather than mixing of sediment from different sources, or sedimentary sorting, to account for the variations in whole-rock c hemistry Development of microscopic foliation-parallel zones enriched in phyllosilicates and in Ti and Al is related to slaty cleavage devel opment and suggests that chemical alteration is coeval with deformatio n and very low grade metamorphism. Volume-loss calculations based on c onservation of ''immobile'' elements, and on deformation of detrital m ica grains, give minimum estimates of approximate to 30%-50% volume lo ss in the most altered shales. The postdepositional changes recorded i n whole-rock chemistry of these shales may be unique to the Ouachita f lysch, but suggest a need for greater caution in the uncritical use of shale chemistry in studies of crustal evolution and provenance.