Changes in Bioclimate Inferred from Palaeosols and Palaeohydrologic Evidence in East-Central Texas

Abstract

Deep red clayey B horizons high in iron have formed in alluvial and colluvial sands and gravels along terraces of Texas rivers flowing eastward toward the coast, possibly indicating that vegetation and climate have changed in east-central Texas. The period of the palaeosol record is indeterminate, but is believed to span from Pleistocene to Recent. Present-day vegetation extends in bands from the Gulf coast to the Edwards Plateau, and includes (east to west) coniferous forests, deciduous forests, blackland prairie and juniper-live oak savanna. Soils under the eastern forested groups are generally moderately acid alfisols developed over sandstones and unconsolidated Quaternary deposits. Towards the west, soils become increasingly basic with a clayey matrix derived from limestones and marls beneath prairie or savanna. Traversing this entire region are ancient paleustalfs formed along rivers flowing towards the Gulf of Mexico. These soils are very acid, dark red alfisols formed from Quaternary alluvium and colluvium beneath mixed forest, grass, and shrub vegetation. Along the Colorado River, the pattern of iron-rich palaeosols extending westward from the forested zone cuts through the coastal prairie zone and extends into the margins of the Texas hill country with its juniperlive oak community. The pattern of soils, their morphology, and their physical and chemical properties suggest that broad gallery forests of pine and oak occupied the river terraces of prehistoric Texas during periods of amenable climate. This interpretation is bolstered by the presence of large stands of pine on sandstone and terrace deposits in Bastrop County far to the west of their normal range. Further support for this position is recorded in alluvial sediments which suggest that palaeo-discharge values were formerly much greater along the Colorado River in the study area. Remnant forests, river sediments, and the palaeosol record may provide evidence which allows the reconstruction of past climatic patterns for a subtropical environment where palaeoclimatic records are rare because of the generally strong weathering regime.