AN INTEGRATED PALEONTOLOGICAL APPROACH TO RESERVOIR PROBLEMS - UPPER CRETACEOUS MEDICINE-HAT FORMATION AND FIRST WHITE SPECKLED SHALE IN SOUTHERN ALBERTA, CANADA
J. Haggart et al., AN INTEGRATED PALEONTOLOGICAL APPROACH TO RESERVOIR PROBLEMS - UPPER CRETACEOUS MEDICINE-HAT FORMATION AND FIRST WHITE SPECKLED SHALE IN SOUTHERN ALBERTA, CANADA, Palaios, 13(4), 1998, pp. 361-375
Sea-level changes in the upper Colorado Group of southeastern. Alberta
, Western Canada, are expressed in the Medicine Hat Formation and Firs
t White Speckled Shale, and can be documented through paleontological
analysis. Presently, the Medicine Hat Formation is actively explored f
or its large, biogenic gas reserves. Combined macrofossil, microfossil
, nannofosssil, and dinoflagellate biostratigraphy reflect a Santonian
age for these units. Fossil analysis indicates that during Late Creta
ceous time the Sweetgrass Arch was a bathymetric high on which a shall
ow marine environment developed within an otherwise relatively deep-wa
ter basin, creating conditions for deposition of Medicine Hat Formatio
n sands. The overlying First White Speckled Shale was deposited during
a time of sea-level highstand during the Niobrara cycle of deposition
. In southeastern Alberta, however, a series of short-lived, relative
sea-level falls during Late Santonian time resulted in siltstone depos
ition. and development of the Sweetgrass Member in the Colorado Group
and similar units on the western flank of the Sweetgrass Arch. During
such events, marginal marine conditions, including possibly increased
input of detrital material or lower salinities, caused a reduction of
nannofossil diversity and near absence of planktic foraminifera. In th
e past, the coarser-grained Sweetgrass Member has been miscorrelated w
ith the Medicine Hat sandstone. Paleontological data analyzed in our s
tudy, however clearly demonstrate a higher stratigraphic position for
the Sweetgrass Member, within the First White Speckled Shale. The base
of the Sweetgrass Member and the boundary between, the First White Sp
eckled Shale and the overlying Mill Creek/Lea Park Formations are mark
ed by erosional unconformities, each of which is overlain by siltstone
and sandstone. Because the Medicine Hat Formation and Sweetgrass Memb
er are thin, paleontological evidence provides a reliable tool for str
atigraphic placement, whereas correlation based solely on lithology ma
y lead to erroneous results.