Palynological Age Determination for Dorcheat and Hosston Formations--Jurassic-Cretaceous Boundary in Northern Louisiana
Robert Rogers
Forty-four core samples from three wells drilled into the Dorcheat and Hosston formations in northern Louisiana were examined for fossil palynomorphs. These samples were obtained from the four beds of the Dorcheat formation (D-1, D-3, D-4), and from the lower two beds of the Hosston Formation (H-1, H-2). A diverse terrestrial and marine palynoflora containing stratigraphically significant species indicates that the cored section of strata spans the Jurassic-Cretaceous boundary.
Fourteen samples in three wells were examined from the lowermost D-1 bed
of
the Dorcheat. A palynological assemblage that is indicative of the Late Jurassic
is recognized on the basis of the presence of the marker dinoflagellate species,
Cleistosphaeridium ehrenbergi and Parvocavatus tuberosus and the absence of
Early Cretaceous palynomorphs.
Four samples were examined from the D-2 bed
of the Dorcheat. These were
barren of palynomorphs.
Diagnostically important palynomorphs indicate the D-3 bed
of the Dorcheat is
Early Cretaceous. Marker dinoflagellates recovered from this member are
Phoberocysta neocomica, Microdinium opaqum, and Muderongia simplex. Early
Cretaceous terrestrial palynomorphs of the D-3
bed
include Trilobosporites sp.,
Pilosisporites sp., Neoraistrickia breviclavata, and Leptolepidites
proxigranulatus. Because the underlying D-1
bed
of the Dorcheat is interpreted
to be Late Jurassic, and as samples collected from the D-2 were barren of
palynomorphs, the Jurassic-Cretaceous boundary of north Louisiana is here
interpreted to be located in the interval between the upper part of the D-1 and
the middle part of the D-3.
The Hosston Formation overlies the Dorcheat and is of Early Cretaceous age.
In addition to Early Cretaceous marker species also found in the D-3 bed
, the
Hosston Formation contains the Early Cretaceous marker spore Cicatricosisporites
angicanalis and the Cretaceous dinoflagellate Oligosphaeridium complex.
AAPG Search and Discovery Article #91042©1987 GCAGS and GC-SEPM Section Meeting, San Antonio, Texas, October 28-31, 1987.