Precambrian alluvial fan and braidplain sedimentation patterns: Example from the Mesoproterozoic Rjukan Rift Basin, southern Norway

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• 作者：Juha Kö ; ykkä ; ^a ; ^{juha.koykka@oulu.fi}
• 关键词：Alluvial fan ; Braidplain ; Geochemistry ; Lithofacies ; Mesoproterozoic ; Rift basin ; Southern Norway
• 刊名：Sedimentary Geology
• 出版年：2011
• 出版时间：1 March 2011
• 年：2011
• 卷：234
• 期：1-4
• 页码：89-108
• 全文大小：4598 K

文摘

An integrated approach of facies analysis, geochemistry and paleohydrology provides new insight into the sedimentology and paleogeography of alluvial sediments in the Precambrian. Here, alluvial fan and braidplain sedimentation patterns are documented in a Mesoproterozoic rift basin, called (known as) the Rjukan Rift Basin in southern Norway. The studied formation (Heddersvatnet Formation) consists of volcanoclastic breccias and conglomerates, cross-bedded sandstones with associated mudstones, and subaerial basaltic lava beds, deposited in a rift basin during the syn-rifting phase. Based on genetically related major lithofacies associations and individual minor lithofacies, the sedimentation is characterized by colluvial and screen apron deposits, subaerial debris flows, hyperconcentrated or sheet flood deposits in the proximal part, representing waning-flood cycles. The distal part contains shallow-water traction current deposits and associated subaerial continental flood basalts that flowed down the rift valleys. The synsedimentary intra-rift faulting formed seasonal or climatically controlled, intrafan ponded-lake with playa lake type cycles and shrinkage cracks.

The geochemical composition of the of the studied sandstones and mudstones suggests a passive rifted continental margin with minor to moderate chemical weathering, possible in a semi-arid/arid paleoclimate. Clastic petrofacies of the sandstones indicate a locally uplifted and syn-rift paleotectonic setting during the main sedimentation stage, passing toward a post-rifting stage of the depositional basin. Small-scale structures within the sedimentary basin affected the depositional patterns of the Heddersvatnet Formation by creating local sites of uplift and erosion, controlling the sediment transport, and defining the accommodation space for deposition. The sedimentation of the Heddersvatnet Formation reflects a tectonic base level fall, which resulted in an accumulation of thick alluvial fan or braidplain sediments near uplifted flank(s) of the sedimentary basin. This was combined with intensive erosion of the basement in the pre-vegetation landscape, which resulted in changes in the hydraulic geometry of flow and sediment discharge of stream channels.

The following criteria, observed from the Heddersvatnet Formation, are suggestive of an ancient alluvial fan and associated braidplain sedimentation: (1) deposition close to the source area; (2) slightly divergent paleoflows; (3) high-energy flows with waning flood-cycles; (4) abundance of poorly sorted and laterally extensive subaerial debris flows, and related streamflows, possible sheetfloods, hyperconcentrated flows, and their associations; (5) great distribution of grain size and lithofacies changes in proximal vs. distal parts; (6) limited suite of sedimentary structures (cross-stratification, ripple-marks, and desiccanation cracks); (7) fault bounded basin (graben) with hanging wall close to uplifted flank(s); (8) paleoslope estimation indicating at least moderate paleotopographic highs; (9) colluvial and scree apron breccias; (10) lack of any typical braided stream channel fill conglomerates and floodplains; and (11) channelized sediment bodies lacking extensive lateral continuity.