Thermal alteration of terrestrial palynomorphs in mid-Cretaceous organic-rich mudstones intruded by an igneous sill (Newfoundland Margin, ODP Hole 1276A)

Abstract

Most approaches used to reconstruct thermal alteration of sediments necessitate advanced, relatively expensive analytical techniques. We have evaluated the fidelity of a less costly, relatively simple approach of visually assessing sporomorph colours to determine thermal alteration. The sporomorph-based thermal alteration estimates were compared to vitrinite reflectance data from the same samples. As study material, we selected a succession of mid-Cretaceous (Albian) organic-rich clay- and siltstones intruded by a diabase sill that was recovered from Ocean Drilling Program (ODP) Hole 1276A, off Newfoundland. Six sporomorph groups (SG), each consisting of morphologically well-defined, easily identifiable constituents with long stratigraphic ranges, were individually evaluated for their thermal alteration signals. These groups are: (1) leiotrilete spores of the genera Biretisporites, Cyathidites, Deltoidospora, Dictyophyllidites, Gleicheniidites, and Leiotriletes (SG-1; subdivided into three subgroups SG-1a, SG-1b and SG-1c with sporoderm thicknesses 1.5 ?m, respectively); (2) trilete, rugulate spores of the genera Camerozonosporites and Lycopodiacidites (SG-2); (3) trilete, striate spores of the genera Appendicisporites, Cicatricosisporites and Plicatella (SG-3); and (4) the gymnosperm-pollen taxon Classopollis torosus (SG-4). Sporomorph colours were determined using Munsell colour standards under reproducible optical conditions. To minimize the potential influence of reworked specimens on the dataset, only the lightest 50% of all counted specimens per sporomorph group were evaluated for their thermal alteration signals. The thermal alteration estimates from all sporomorph groups yield an internally consistent picture that is compatible with vitrinite reflectance data from the same samples. They indicate that downhole thermal alteration does not increase until 20 m above the igneous sill. A steep rise occurs only at 4.23 m above the sill, and thermal alteration peaks in the sample closest (2.17 m) to the sill. However, the different sporomorph groups exhibit varying degrees of fidelity with respect to deciphering thermal alteration. Factors influencing the precision of the thermal alteration signal include sporoderm thickness, character of surface ornamentation, resistance to reworking, and abundance in the sample material. Highest correlations with vitrinite reflectance data are observed for the thermal alteration values from SG-1b (R=0.82), SG-3 (R=0.80) and SG-4 (R=0.80). Hence, these groups are best suited for a sporomorph-based approach to reconstructing the thermal history of sediments. The highest correlation coefficient with vitrinite reflectance data is registered for SG-1b, the subgroup with the least variability of sporoderm thickness and the highest abundance in the sample material. This indicates that the study of morphologically similar, highly abundant specimens with strongly constrained sporoderm thickness variations yields the best results for the reconstruction of thermal alteration.