Discriminating
Depositional Habitat and Provenance of Mudrock Through Use of Spectral Gamma Ray
Paxton, Stanley T.1, Anna M.
Cruse2, James O. Puckette2, Michael Aufill2
(1) 1009 Woodcrest,
Spectral gamma ray (SGR) provides
detailed information about sedimentation, depositional environment, and
provenance of mudrock. For the present study, SGR
data were sampled at locations throughout
The analysis suggests that SGR of mudrock deposited under anoxic and/or euxinic
conditions is dominated by U (organic-rich, pyrite-bearing, and finely
laminated). The abundance of U in black shale (and hence, the strength of the
gamma-ray signal) is attributed to the duration of starved sedimentation on the
ancient seafloor (which favors diffusion and fixation of U in the substrate).
SGR measurements that plot close to the U pole on the triangle represent
organic-rich “condensed” sections. In contrast, mudrock
that is relatively enriched in K and/or Th is
interpreted to have accumulated under more oxic
depositional conditions. K-rich mudrock is suggestive
of derivation from relatively unweathered provenances
with sedimentation rates >10 cm/ky.
Th-rich mudrock is
characteristic of a provenance or depositional setting that has undergone deep
weathering or leaching to produce a “residuum”. In theory, some Th-rich mudrock may represent an
"organic-poor" condensed section.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California