Stratigraphy, Paleoenvironments and Thermal Maturity of
Cambrian Strata, Reelfoot Rift, Southeastern Missouri: A Case Study of
Integrated Palynology, Sedimentology and Geophysics
Gordon D. Wood, Rick C. Tobin, J. Todd Stephenson
Regional stratigraphic, paleoenvironmental and thermal maturity
interpretations of Cambrian strata in the Reelfoot Rift area were enhanced
through the integration of palynologic, sedimentologic and seismic data.
Outcrops in the St. Francois Mountains of Missouri were compared with mineral
cores and exploration
wells in southeastern Missouri and northeastern Arkansas.
This comparison indicates that paleoenvironments range from inner ramp
(outcrops) to middle ramp (mineral cores) to distally steepened ramp margin to
basin (deep
exploration
wells). All productive palynological samples from this
area were dominated by clusters, filaments, and "sheets" tentatively considered
algal in origin. Acritarchs recovered can be assigned to Elektoriskos,
Granomarginata, Leiosphaerid a, Lophosphaeridium, Micrhystridium, Timofeevia,
Vulcanisphaera and several new forms. This assemblage is the first record
of Granomarginata squamacea Volkova 1968, Timofeevia phosphoritica
Vanguestaine 1978, and Vulcanisphaera turbata Martin in Martin and Dean
1981 from the Upper Cambrian warm-water provincial realm. Synchronous sequence
boundaries were established using palynological "extinctions". These horizons
were used to correlate age-equivalent strata from inner ramp to basin. In a
regional context there is a slight increase in the abundance and diversity of
organic-walled microphytoplankton from nearshore to offshore environments.
Anomalies in palynological distribution trends can be explained using geological
and seismic evidence.
Regional patterns in thermal maturity were also determined through the
integration of palynology and fluid inclusion thermometry. A visual
determination of thermal maturity and its relationship to dip location and
burial history was assessed using various organic-walled microfossils. For
example, the color of Timofeevia phosphoritica recovered from outcrop
samples is colorless-to-light yellow, indicating low maturity (pre-oil window).
In shallow mineral cores, this species is yellow-to-light brown (oil window),
and in samples from deep penetrations it is dark brown-to-black (gas window).
Other organic-walled microfossils (e.g., algal clusters, filaments, etc.) also
exhibit this same color trend. Fluid inclusion homogenization temperatures were
also utilized to help constrain hermal maturity interpretations. Fluid inclusion
data were derived from dolomite and calcite host minerals taken from exploration
wells. These data were tied to petrographic observations to elucidate regional
tectonic history and facilitate isopaching of maturation indices.
AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995