RELATING PALEOBOTANIC, PALEOECOLOGIC, AND PALEOCLIMATIC VARIATION IN THE PENNSYLVANIAN-PERMIAN MARKLEY FORMATION OF NORTH-CENTRAL TEXAS

The Pennsylvanian-Permian boundary in the western Pangean tropics records a major change in the composition of terrestrial floras. At a coarse level of time-space resolution, the change is from floras dominated by wetland plants (humid-to-subhumid climates) to those dominated by plants typical of subhumid-to-semiarid climates. At a finer scale of resolution, it can be seen that these floras are intercalated, even at the outcrop scale, and that the change from one to the other appears to represent a shift in the prevailing climate regime. This vegetational change is well represented in the Markley Formation (Uppermost Pennsylvanian-Lowermost Permian) of north-central Texas. The Markley Formation consists mainly of channel sandstones and paleosols. However, it also includes shallow incised channels within which plant-bearing shales and sandstones are preserved. These fossiliferous deposits are significant due to their range of lithologic variability, which appears within recurring stratigraphic patterns as follows: Deeply-rooted basal paleosols; immediately superjacent kaolinitic-quartzitic mudstones rich in seed-bearing plants such as sphenopterids and conifers; overlying organic shales rich in “Pennsylvanian”-type lycopsids and pteridosperms; overlying floodplain mudstones that contain a highly diverse flora distinct from the underlying organic shales. These floodplain mudstones are often capped by sparsely fossiliferous fluvial sandstones that contain Cordaites foliage. Because floral composition is linked to lithologic changes, samples could be grouped into six distinct categories. Each lithologic group contains a distinct floral assemblage, consisting of well-defined dominant taxa and various groupings of accessory taxa. Recurrent patterns throughout the Markley Formation indicated that semiarid-subhumid vegetation was preserved most commonly at the base of the shallow incisions, and was succeeded in time by humid climate wetland assemblages. Furthermore, temporal changes were observed in the composition of dominant taxa within particular lithologies, which gives evidence of dominance shifts/replacements in a given environment over geologic time.