Sedimentary
Facies and Architecture of a Subtidal Sandbar Complex, Lower Cambrian Gog
Group,
Desjardins, Patricio R.1, Luis
A. Buatois1, Brian R. Pratt1, M. Gabriela Mangano1
(1)
During the Cambrian, sedimentation on the
continental margin of western Laurentia evolved in a context of worldwide
transgression. Thus the architecture, distribution and facies of sandstone
bodies in the Gog Group provide important clues towards a better understanding
of sand dynamics on a broad, transgressive continental shelf. Our study focuses
on the lower portion of this unit which reaches some 320 m in thickness. This
study is based on the integration of lithofacies and ichnofacies analysis,
paleocurrent measurements, interpretation of photomosaics, and correlation
between sections. Four units were recognized: 1) tabular bodies of planar and
trough cross-stratified sandstone; 2) tabular to lenticular bodies of
cross-stratified and plane-laminated sandstone intercalated with wavy to
tabular packages of ripple cross-laminated sandstone and shale; 3) relatively
thick, nearly tabular bodies of planar cross-stratified sandstone with
domed-shaped tops; and 4) interbedded lenticular to wavy packages of sandstone
and shale. Units 1 to 3 reflect strong uni-directional currents within
sandbars. The lenticular geometry of some of the beds reveals storm reworking,
but the reactivation of uni-directional currents may have erased most of the
signals of wave action in specific areas. Protected areas or lagoons (unit 4)
developed as an associated subenvironment. Two different ichnofacies were
identified and characterized: 1) Skolithos ichnofacies within units 1 to
3; and 2) Cruziana ichnofacies within unit 4. These deposits record
growth and migration of a subtidal sandbar complex over a low-gradient shelf
above storm wave base. The Gog Group represents an instructive outcrop analogue
for transgressive subtidal sandstone reservoirs.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California