Journal of Petroleum Geology, Vol.11,
No.3, pp. 325-340, 1988
©Copyright 2000 Scientific Press,
Ltd.
THRUST CONTROL ON THERMAL
MATURITY OF THE FRONTAL OUACHITA MOUNTAINS, CENTRAL ARKANSAS, USA
M.B. Underwood(1), D.A. Fulton(1,2)
and K.W. McDonald(1)
1 Department of Geology,
University of Missouri, Columbia, MO 65211, USA.
2 Present address: Texaco, USA, PO
Box 60252, New Orleans, LA. 70160, USA.
Abstract
Three types of anomalies in thermal
maturity exist within the frontal thrust-belt of the Ouachita
Mountains, Arkansas, USA. The anomalies were documented by
combining structural mapping with measurements of vitrinite
reflectance; all samples were obtained from surface exposures.
Maturity-inversion occurs along the Y-City Fault, where Jackfork
and Johns Valley strata (Lower Pennsylvanian) have been thrust
over less-mature sequences of the lower Atoka Formation (Middle
Pennsylvanian). An apparent shear-heating aureole is also
associated with this important thrust, although the spatial
extent of the aureole is limited. When allowances are made for
both structural and stratigraphic position, we find that internal
gradients in vitrinite reflectance within individual
thrust-sheets are abnormally low. We attribute these gradients to
tectonic burial. Weak remnants of the primary stratigraphic
gradients are retained in most instances, so organic metamorphic
reactions evidently failed to equilibrate during thrust-loading;
rapid unroofing of the thrust-belt may have begun even as some
faults remained active. Similar patterns of maturation should be
expected within other thrust-belts where peak burial and heating
events were synchronous with deformation. Under these
circumstances, subsurface projections of maturation trends should
be keyed to structural (rather than stratigraphic) position.