Miocene Synvolcanic Alluvial Sedimentation in Lignite-bearing Soma Basin, Western Turkey

Calcalkaline volcanism and volcaniclastic deposition controlled considerable mode of late Miocene sedimentation and basinal development of the lignite-bearing Soma basin in western Turkey. The volcanism-induced Denis Formation, overlying the Soma Formation discordantly, is represented by two rock assemblages: 1) In the depositional axis of the basin, green, fine-grained sandstone dominated alluvial-lacustrine deposits and, 2) In the northern margin of te basin, syn-eruptive volcaniclastic apron deposits which are mainly composed of fine and coarse-grained volcaniclastic/pyroclastic rocks and lavas. The primary volcanic rocks surrounding probably low-relief volcanoes representing near-vent assemblage are composed mainly of andesitic, rhyolitic and basaltic lavas and pyroclastics. These volcanic rocks show a complex transformation to syn-eruptive debris flow and hyperconcentrated stream/flood flow processed volcaniclastic apron deposits. The debris flow dominated volcaniclastic apron deposits extend as much as 20-25 km from the multi-vent and low-relief volcanoes. The gradational lithofacies assemblages within the volcaniclastic apron deposits show a proximal to distal change from massive, coarse volcaniclastics to thickly-to thinly bedded, fine-grained ones, and from poor channelized to unchannelized flows. From proximal to distal parts of the apron system, the abundance of debris flow deposits decrease and of hyperconcentrated flood-flow deposits increase gradually. Pyroclastic fallout products like pumices representing explosive volcanism, were spread widely within the basin. Due to the rapid sedimentary processes, huge amounts of unconsolidated volcanic detritus were filled probably fault-controlled topographic lows of the basement rocks and green fine-grained alluvial deposits. The distal volcaniclastic apron deposits are represented coarse-to fine-grained tuffaceous sandstones and they are disturbed or cutted by subvolcanic bodies and covered locally by thin brecciated basalt lavas.