The Importance of Fractured Olistoliths and Shelf-Gravel Sorting for the Construction of a Tectonically-Controlled Carpathian Margin, Albian Bucegi Conglomerates, Eastern Carpathians, Romania

Olariu, Cornel; Jipa, Dan C.; Steel, Ronald J.; Ungureanu, Costin

The Bucegi Conglomerates are interpreted as a tectonically active Carpathian margin with a narrow shelf to deepwater slope morphology. The spectacular 2, 000 m thick conglomerate outcrops of the Bucegi were initially interpreted as large alluvial fans, but later reinterpreted as coarse-grained deepwater slope deposits in an influential article by Stanley (1978), at a time when studies of modern and ancient deepwater systems were just beginning to take off. We add to Stanely's observations by interpreting the uppermost third of the Bucegi succession as shelf deposits. Despite the narrowness of this shelf it was an important staging area where huge fractured limestone olistoliths were shed from the tips of thrust faults, where fractured limestone clasts were further broken up, and where coarse sediment was partially sorted by currents and waves. These shelf processes can be inferred from the sorted grain populations now identified on the deepwater slope. In the basin margin reconstruction we propose a narrow shelf backed by a thrust system across which a fluvial system with large limestone olistoliths (toward N and NW) that passed basinwards to a shallow marine area toward south and south-east. The rivers and shallow marine processes transferred the coarse material at times directly onto the slope which was a hundreds of meters high (based on the presence of large clinoforms). Deep basinal deposits coeval with the shelf and slope conglomerates have not been described but the finer Bobu and Teleajen "flysch" deposits that have been interpreted as the same age turbidite deposits and are potentially genetically linked with the Bucegi conglomerates.

The Bucegi shelf was most likely by-passed by sediment during the largest river floods which delivered the coarsest (conglomerate) and most poorly sorted sediments onto the upper slope. During normal discharges gravel and sand were stored on the shelf, were subject to the significant sorting by waves and current, and these deposits too were delivered to the slope in periods of flooding. The slope deposits thus show a wide range of poorly to well-sorted, grain populations, including extremely thick (3 to 5 meter beds) inverse graded beds interpreted as highly mobile debris flows. Compared with other margins, the Bucegi margin was extremely conglomeratic because of its proximity to the tectonically active mountain range, the huge gravel-rich river discharges and the narrow (10-20 km) shelf.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013