Genetic significance of an Albian conglomerate clastic wedge, Eastern Carpathians (Romania)

The impressive 2000 m thick conglomerates of the Bucegi Formation exposed in the southernmost part of the Eastern Carpathians were interpreted initially as large alluvial fans, and later suggested to be deposited as deepwater submarine-slope deposits. However, the routing system of the coarse sediment transfer from the source area to the deepwater slope (source-to-sink analysis) has not been explained and the mechanisms involved in the shelf sediment storage and bypass onto the slope have not been discussed.The present research on the Albian Bucegi Formation has provided the following new insights on their source-to-sink aspect: (1) that the Upper Member of the Bucegi Formation, with its frequent channelized and sheet like fine conglomerates and sandstones, contrasts greatly with the Middle and Lower members of deepwater slope and basin-floor origin. The Upper Member is interpreted as fluvial and shallow-marine deposits that were temporarily stored and reworked on a ‘shelf’, albeit a narrow one, bridging the area between the deforming hinterland and the deepwater slope deposits; (2) the Upper and Middle members are genetically linked and developed through the basinward migration of a large-scale (hundreds of metres in amplitude) clinoform with relative flat-lying topsets and slightly steeper (few degrees), coarser grained slopes that built out to the south and southeast; a configuration that is common along continental margins and also generally along all types of deepwater basin margins; (3) the Middle Member contains a range of submarine, sediment density flows that vary from high-density, mobile debris flows to lower-density sandy turbidites. The sediment textures (sorted grain populations) inherited from the shelf ‘sorting factory’ can to some extent still be recognised in the slope stratigraphy; and (4) the large (10–20 m diameter) carbonate and metamorphic olistoliths that are ubiquitous on the shelf and (to a lesser extent) slope, reflect the steep gradients and very active tectonic setting of the fractured and thrusted hinterland, from which these outsized blocks were transported onto the adjacent shelf.The now-proposed, narrow shelf platform of the Albian Bucegi basin margin thus functioned to temporarily store sands and gravels, to distinctly sort some of this sediment, and to eventually bypass both sorted and new flood-generated, unsorted materials onto the slope. Compared with other basin margins, this Albian Bucegi margin was extremely coarse grained because of its proximity to the actively deforming mountain range, to a fractured basement that produced more gravel than sand, to the great sediment flux from steep short rivers, and to the narrowness (10–20 km) of the shelf.