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.