A shallow marine volcaniclastic facies model: an example from sedimentary rocks bounding the subaqueously welded Ordovician Garth Tuff, North Wales, U.K.

Abstract

Volcaniclastic sedimentary rocks bounding the Ordovician Garth Tuff in North Wales were deposited in a shallow marine setting adjacent to a magmatic arc. The volcaniclastic sedimentary rocks are primarily of angular to euhedral quartz, angular euhedral feldspar, and volcanic rock fragments. These grains are of a pyroclastic origin, but have been reworked to various degrees. Sedimentary rock fragments, rounded quartz, muscovite, biotite, iron-rich chlorite and various clay minerals occur in lesser amounts. The sedimentary rocks can be divided into a proximal offshore to foreshore facies association (POFFA) and an offshore to lower shoreface facies association (OLSFA). The sand-dominated POFFA, exposed near Capel Curig, is characterized by large wave ripples, hummocky cross-stratification, co-sets of trough cross-beds, reactivation surfaces, plane beds, and graded turbidite layers in suspension-deposited mudstone. Water depths calculated from bedding plane exposure of large wave ripples and determined by the presence of hummocky cross-stratification and by facies association vary from a few meters to around 30 m (45–50 m theoretical maximum) in a 45 m-thick stratigraphic section beneath the tuff. Eight kilometers to the southeast, the Garth Tuff is bounded by a thick sequence of suspension-deposited laminated mudstone with thin graded beds of siltstone and fine-grained sandstone. This facies association (OLSFA) represents the introduction of material by distal turbidites and by pelagic sedimentation. The OLSFA was deposited in water depths well below storm wave base, possibly in excess of 200 m. No evidence of storm waves or surface-generated currents occur until over 40 m above the tuff. This study documents marine sedimentary rocks, deposited in water depths ranging from foreshore to offshore, as the bounding facies of the Garth Tuff. It is, thus, reasonable to conclude that the Garth Tuff was emplaced and welded in water depths greater than the thickness of the tuff in the area of this study.