Micromorphology and Sequence Stratigraphy of the Interfluve Paleosols from the Ganga Plains: a Record of Alluvial Cyclicity and Paleoclimate During the Late Quaternary

Abstract

Thin-section details of 56 paleosol-forming intervals in four successions along ~ 75 km width of the Ganga–Yamuna interfluve (GYI) show varying intensity of pedogenesis and distinct changes in key paleopedogenic features over the last 100 ka. Two 50-m drill-core sections located in the marginal parts of the GYI are characterized by dominance of weakly developed paleosols and two Bt horizons. In contrast, in the middle part of the GYI, one 32-m drill core and one 11-m cliff section show predominance of well-developed paleosols and three or four Bt horizons. These paleosols are similar to modern Entisols, Inceptisols, Alfisols, and Vertisols. The paleopedofeatures of the paleosols indicate at least three phases of humid conditions spanning 90–80 ka, 50–30 ka, and 10 ka with intervening drier conditions. The paleosols formed during the humid phases are defined by extensive illuviation, increased mineral weathering, and strong pedogenic activity. Dominance of pedogenic carbonates and weak pedogenic development suggest drier conditions during 75–60 ka and 30–20 ka. Alluvial cyclicity of the GYI successions based on intensity of pedogenic development and presence of Bt horizons shows differences in the fluvial aggradational cycles (FACs), FAC sets, and the sequence boundaries (Sb) on account of allogenic and autogenic controls. Two major sequence-boundary features at ~ 80 ka and ~ 10 ka are defined by the formation of Alfisols over the entire GYI. They define large-scale degradation, hiatus in sedimentation, and incision across the entire interfluve over the last 100 ka. In addition, a few more boundary features of lesser-than-average lateral extent developed only in the middle part of the GYI during 50–30 ka. This was caused by a progressive uplift, incision, and tilting of the interfluve during this period. We demonstrate that the mature paleosols in fluvial strata of the inland alluvial valleys like the Ganga Plains can serve as useful stratigraphic markers to describe the nonmarine sequences.