Sedimentology and paleopedology of Miocene alluvial deposits at the Paşalar Hominoid site, Western Turkey

Abstract

Miocene age alluvial deposits at the Paşalar Hominoid site in Western Turkey consist of a fossil-bearing green sand unit and two drab-colored, calcareous paleosols that sandwich the fossiliferous bed. The green sand unit contains a significant amount of bone chips and teeth. High concentrations of gravel, bone and teeth occur in, and directly above, large pothole shaped bodies which extend from the green sand down into the underlying calcareous paleosol. Gravel pipes emanating upward from the vicinity of these “potholes” are interpreted as fluid escape structures. The fossiliferous green sand bed is interpreted as the deposit of a silty, pebbly sand flow ( en masse emplacement) on the basis of its overall normal grading, vague horizontal layering, grain and matrix supported fabrie, and abundant fluid escape structures. The enigmatic potholes may have been holes produced by the burning out of tree trunks in the soil; they match modern examples well. The two calcareous paleosols, termed the lower and upper calcareous silt, are very much alike and represent a soil-forming environment where both clay formation and calcification took place. Pedogenic features common to both paleosols include green colors, high clay content, clay skins and clay slickensides, an unusually great thickness (about 2 m after compaction) and an abundance of golf-ball-sized calcareous nodules. The gleyed nature of the paleosol indicates a climate with seasonal waterlogging. Abundant calcareous nodules in the paleosol profiles indicate a pronounced dry season and clay formation indicates a warm, humid season. Up-section from the fossil beds is the main alluvial sequence of thick non-calcareous siltstones and cobble conglomerates exposed in gullies to the northeast of the exacavation site. The siltstones are pervasively mottled red and green. They have abundant soil structures including root traces, blocky peds and cutans. These massive, mottled siltstones are interpreted as paleosols of a generally well drained floodplain and the interbedded conglomerates are interpreted as braided stream channel deposits. The main alluvial sequence has dips of less than 3° whereas the fossil beds and their clayey paleosols dip between 7° and 10°. Thus there was an episode of tilting between the deposition of the two sequences. A riverine levee and floodplain depositional environmental is interpreted for the Paşalar fossil beds based on the gleyed paleosol profiles and the onlap of channel fill and levee deposits on both the fosiliferous sand unit and upper calcareous silt. They probably formed in a low-lying stream side location in a wet-dry monsoon climate, similar to semi-arid to semi-humid areas in the Punjab province of India. On the basis of comparison with modern soils from the Punjab, similar Paşalar paleosols are classified as intermediates between Calciorthids and Ustochrepts that have gleyic properties.