Интерпретация строения пойменного аллювия: на примере разрезов геоархеологического объекта Остров Лиственичный (Северо-Восточное Приангарье)

Abstract

This paper examines characteristic features of floodplain accumulation at geoarchaeological site Ostrov Listvenichnyi, located on the Angara River at the same-name island (Northeast Angara region, Baikal Siberia). The problems of interpretation of topographic and lithological data, island architecture, glacial and postglacial natural climatic insights (MIS 2–MIS 1) are also touched upon. The accumulation of sediments at a series of 16 archaeological test pits recorded along the northwestern side of the island was analyzed by using pedolithological method. Island formation history included the upbuilding of the origin island based on river point bar, further transformation into “east” pre-island and final articulation of ancient “east” pre-island and younger “west” pre-island. The primary differences in alluvium composition of ancient pre-island (red beds, clay loam) and Holocene sedimentations (grey beds, sandy loam), both situated at the same level, were identified and explained. Detailed analysis of floodplain accumulation revealed eight distinct lithological layers varying by the structure and the composition. The following conclusions were reached based on the data available: lithological strata 1–5 have formed in Sartan, lithological strata 6–8 have formed in Holocene. Every layer contains the information on the changing climate and environment: signals of floods (high flood stages in 7 layer, “dry” stages in 3 layer), different phases of humification (humusless strata 1–4, first fragile humus horizons in layer 5, more pronounced humus horizons a, b, c, d in layer 4, and humus background in strata 7–8), epigenetic markers of сryogenic processes in strata 3–5. Analysis suggest also three chronologically differentiated floodplain benches: lower 1,5–2 meter bench (top of layer 3) associated with Middle Sartan; middle 2,5–3 meter bench (top of layer 5) associated with Final Sartan; top 4,5–5 meter bench (top of layer 8) associated with modern time.