Rhyolite Magma Intrusion into Subaqueous Unconsolidated Sediments: Facies, Structures and Formation Processes

Abstract:

The Late Miocene to Pleistocene ignimbrites and related caldera morphology are widely distributed to the backbone range of the northeast Japan Arc. In the middle Tohoku, Kawasaki Town, Miyagi Prefecture, the late Miocene to Pliocene lacustrine deposits (of caldera lake origin) with rhyolite intrusions are well exposed and have a variety of litho- and sedimentary facies (7 units). Unit A is massive and composed of fine sand to very fine sand sized vitric ash. Unit B consists of concentrated blocks of pumice of mms to 10 cm in size, occasionally with 50 cm to 1 m in diameter. Unit C1 comprises concentrated blocks of mms to 20 cm sized, brecciated granite and pyroclastic rocks and obsidian fragments with mms to 3 cm in diameter. Unit C2 is of well-stratified volcaniclastic deposits comprising mm sized poorly vesiculated pumice and subangular obsidian fragments and very fine to fine sand sized vitric ash. Unit D consists of fine to very fine sand sized vitric ash. Unit E is a 30 cm to meter-sized block with spherical morphology. Blocks, comprising fragments of mms to 20 cm sized pumice, are coated by cm to dms thick vitric layers. Pumice is subangular to angular in shape and fibrous and thin cooling joints are present. Unit F is composed of silt, very fine sand to medium sand sized vitric ash and silciclastic sand. The deposits contain pumice, accretionary lapilli, lithic fragments, and charred wood fragments. Locally the deposits erode the underlying units. These units are suggestive of series of magma intrusion processes into subaqueous unconsolidated sediments (Stage 1 to 4). Stage 1: Unconsolidated lacustrine deposits (reworked ignimbrite) underwent injection of rhyolite magma. Soft sediments with ductile deformations were trapped in the Unit B deposits. Stage 2: The Unit C pyroclastic dyke intruded to the Unit A, B sequences. When the dyke reached out the lake bottom surface, it turned to a small-scale subaqueous eruption resulting in pyroclastic units of pumice and obsidian fragments. Stage 3: Pyroclastic intrusion (Unit E) occurred to the Unit D deposits. At the tips of intrusions rhyolite magma broke apart into large spherical aggregates. Stage 4: Volcaniclastic sediments deposited in a lacustrine deltaic environment (Unit F), as the part of lake was filled with volcaniclastic sediments derived from an intra-caldera catchment.