Abstract

Pangea represented an exceptional global paleogeography, characterized by a single supercontinent symmetrically disposed about the equator. This configuration had an extraordinary effect on global paleoclimate. The Permian to Triassic sedimentary section, with its distinct sedimentologic facies, provides important evidence of a unique global monsoonal climate. Historically, paleoclimate inferences have been attributed to red beds, but ferric oxides that color the strata form in early- to late-diagenetic oxidizing Eh-pH conditions that occur in a spectrum of wet to arid climatic settings. Thus the red color of these beds has no particular paleoclimate significance. However the assemblage of sedimentary and biogenic structures and depositional facies in both red beds and non-red beds provide criteria to assess the distribution, amount and frequency of precipitation and, ultimately, paleoclimate.

Criteria that reflect the depth of the paleo-water table are keys to interpreting paleoclimate in Pangean marginal-marine and continental red beds. Paleo-water tables may be locally controlled by tectonic setting, drainage, or seasonal precipitation and temperature variations. Features present within geographically and temporally widespread strata provide the best climatic indicators. In playas, lakes, floodplains and tidal flats deep and wide desiccation cracks indicate previously water-saturated sediment, whereas thoroughly dry sediment that is subsequently only partially saturated will produce short, narrow, complex cracks commonly associated with vesicles. Evaporites in groundwater-saturated saline mudflats exhibit vertical decrease in crystal size and possibly changes in mineralogy and may be associated with sedimentary fabrics indicative of thick, efflorescent salt crusts. Evaporites formed in dry soils in arid or monsoonal-dry settings typically show an upward increase or no variation in crystal size and character, may be associated with rhizoliths and show no preference to depositional setting. Wind deflation and extensive eolianites related to restricted vegetation may indicate deep or saline paleo-water tables. Paleosols and pedogenic features such as mottling, nodules and soil fabrics may overprint any subaerially exposed continental or paralic strata and indicate paleo-water table position, fluctuation and degree of saturation. Biogenic structures such as rhizoliths and invertebrate ichnofossils can mark the sediment-water or wet sediment-air interface, or the vadose and phreatic zones.

Sedimentary fabrics and biogenic structures are syndepositional features and are independent of rock color imparted by subsequent diagenetic alterations. These criteria, integrated with paleontologic data and compared to predictive climate models, are the basis for interpreting paleoclimate.