Depositional environments and diagenesis of the Cenozoic sediments of the Almazan Basin, North-East Spain

• Main Author: Bond, J.
• Published: University of Cambridge 1989
• Subjects: 551.7
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.596756

The Almazan Basin is an half-graban, which formed between the Sierra de la Demanda and the Celtiberian Range. It is 30km-55km wide, 115km long and contains up to c.3.5km of Tertiary alluvial and lacustrine sediments. The spatial and temporal distribution of these deposits during the basin evolution is described. A lithostratigraphy is proposed which subdivides the Cenozoic sediments according to lithology, biostratigraphy and depositional unconformities. The Almazan sediments are assigned to the Henar Group (Eocene-Oligocene) and Jalon Group (Late Oligocene-Miocene). The Henar Group, comprising steeply dipping strata outcropping along the NE basin margin, is subdivided into alluvial and lacustrine sediments of the Tapiela and Deza Formations and alluvial sediments of the Gomara Formation. The Jalon Group, which includes flat lying sediments of the central and southern basin, comprises alluvial sediments of the Jaraba, Ariza, Ibdes, San Roquillo and Somaen Formations and lacustrine deposits of the Arcos de Jalon Formation. Nine alluvial fans deposited sediment in the Almazan Basin. Two types of fan morphology are recognised: small alluvial fans fringed the fault-bounded basin margins and large, 'humid' alluvial fans were sourced from the corners of the basin. Fan size and alluvial sedimentary processes were influenced by structural setting, the size and geology of the catchment area and climate. Basin-fringing fans had small catchment areas, draining Mesozoic limestones, which promoted the formation of stream and debris flow dominated alluvial cones. In contrast, the large 'humid' fans drained wide catchment areas, of mixed sandstone and limestone terrains, and supported perennial rivers. The Jaraba Fluvial System (?Oligo-Miocene) for example, was derived from the SE corner of the basin and flowed north-westwards depositing conglomerates, channel and sheet sandstones, and overbank fines. A catena of palaeosols developed on the Jaraba fan. Three lacustrine units are preserved. The Tapiela Formation comprising gypsiferous marls was deposited in a saline lake. The Deza and Arcos Formations comprising fossiliferous and nodular limestones, dolomicrites and mottled sandy limestones, were deposited in ephemeral lakes located on the distal floodplains of the Gomara and Jaraba Fluvial Systems. Cyclic lake sedimentation occurred on two scales. Large scale shifts in the pattern of lake sedimentation were caused by prolonged climatic changes lasting tens of thousands of years. Small scale, transgressive and regressive depositional cycles are attributed to autocyclic factors causing minor fluctuations in palaeolake level and chemistry. Stable isotope analysis of the lacustrine limestones of the Deza Formation reveals increases in the carbon-13 and oxygen-18 composition of the dissolved inorganic carbon, in association with peaks of lake productivity. Periodic desiccation of Lake Deza is inferred by the accumulation of dolomite, and the accompanying fractionation of carbon-13 and oxygen-18 values. Clay analysis by XRD reveals two clay mineral facies. The detrital clay minerals mica, kaolinite and chlorite occur in alluvial lithofacies. In contrast, an authigenic suite of magnesian clays is present in the lacustrine limestones. Supralittoral dolomitic limestones contain illite and palygorskite whereas littoral limestones contain illite, smectite and kaolinite.