Calcretes, oncolites, and lacustrine limestones in Upper Oligocene alluvial fans of the Montgat area (Catalan Coastal Ranges, Spain)

The Chattian (Upper Oligocene) deposits of Montgat consist of continental detrital sediments deposited mainly in alluvial fan environments. Stratigraphic and petro-graphic data allow identification of two lithostratigraphic units: the Turó de Montgat unit and the Pla de la Concòrdia unit. These units are interpreted as two coalescent alluvial fans deposited synchronously. The catchment areas of these alluvial fans were located between the Collserola and Montnegre highs and consisted of a Mesozoic cover overlying a Paleozoic basement. Intrabasinal limestones interbedded in the two alluvial fan deposits of the Montgat area have been analyzed geochemically, and three groups are distinguishable. The first group is composed of the oncolites of the Turó de Montgat unit, which were formed in ponding zones in the channel or disconnected pools in a fluvial setting where waters remained under closed conditions. The second group is composed of the lacustrine limestones and the oncolites of the Pla de la Concordia unit, which were formed in an open and permanent fluvial setting. The third group is composed of the palustrine limestones and tufa-oncolites of the Pla de la Concordia unit; these were formed in ephemeral fluvial settings concurrent with development of calcrete soils. The geochemistry of the intrabasinal limestones deposited in the Chattian alluvial fans of the Montgat area is mainly controlled by the fluvial regime and the lithology and altitude of the catchment areas and the sedimentary basin. © 2006 Geological Society of America.