Alluvial Phases Research Articles

Late quaternary alluviation and soil formation in the southern Argolid: its history, causes and archaeological implications

In the context of an archaeological survey of the southern Argolid, Greece, studies have been carried out to elucidate the evolution of the landscape since its earliest known human occupation about 50,000 years ago. One of these studies was a detailed geological mapping of the late Quaternary alluvium and soils in the area. Dated by means of thorium-uranium disequilibria, archaeological finds, and historical information, seven periods of alluviation were identified, each of short duration relative to long intervening periods of stability and soil formation. The three earliest alluvial phases, falling before and during the last glacial interval, range from about 330,000 to 32,000 years in age. No alluviation accompanied the last glacial maximum around 20,000 years ago. In fact, a stable landscape persisted until about 4500 years ago, when debris flows and widespread aggradation in the valleys resulted from major slope destabilization and soil erosion, probably as a result of extensive land clearance in the Early Bronze Age. A subsequent stable period lasted through the many upheavals of the later Bronze Age, the Dark Ages, and the early historical period. It came to an end with a brief phase of alluviation between about 300 and 50 BC. Stability returned through the late Roman period, notwithstanding considerable expansion of the settled area. Another period of destabilization, this one marked by debris flows and hence major soil erosion, is poorly fixed in time, but probably coincides with expanded maquis clearance accompanying the resettlement of the area around AD 1000. Subsequent events of soil erosion and aggradation vary in nature and timing from one drainage to the next and, in some areas, continue today. Nature and chronology of the soil forming and alluviation events show that simple correlations with climatic events do not suffice to explain them. For the latter ones, past about 2500 BC, human activity seems to be the dominant cause, but once again the relation between cause and effect is not straightforward. Land clearing, or neglect of soil conservation efforts during economic downturns, appear to have a more devastating effect upon the landscape than do intensive land use or total land abandonment.

Fluvial Sedimentation Conditioned by Quaternary Climatic Change in Central Texas

ABSTRACT The Colorado River of Texas drains a relatively low-relief basin of diverse geology and climate. Detailed geomorphic mapping of Colorado River flood plain and paleochannel morphology on the Texas Coastal Plain shows channel metamorphosis through the late Quaternary. The coarser sediment of the low-sinuosity river phases is relatively poorly sorted, lacking in silt and clay, and showing a ratio of C to M ranging from 12-80. Sedimentary properties are consistent with the fluvial regimen of a relatively arid climate. The high-sinuosity phases transported mostly silt, clay, and fine sand during relatively humid phases when interfluves were stabilized by thick soils and ample plant cover. The altering hydrologic regimen of the Colorado River during the Quaternary has resulted in long perio s of storage for alluvium in flanking terraces. With increasing time for weathering the alluvium is first enriched in plutonic rock fragments plus silica, and subsequently in siliceous clasts alone. During arid climatic phases these deeply weathered suites were introduced into the active channel alluvium. This contamination may result in anomalous roundness-versus-size relationships. The low-sinuosity, arid alluvial phases all have extremely well-rounded quartz associated in any given size class with poorly rounded limestone and chert and even with poorly rounded caliche fragments. The quartz rounding derived from multicyclic storage, weathering, and transport, while the softer components were more recently introduced from outcrops. Arid-phase alluvium also tends to have coarser partic es of a given lithology not necessarily better rounded than finer ones. Humid alluvial phases of the Colorado River show more normal relationships between roundness, lithology, and grain size.