Processes of magma wet sediment interaction in a large-scale Jurassic andesitic peperite complex, northern Sierra Nevada, California

The Middle Jurassic Tuttle Lake Formation in the northern Sierra Nevada, Ca lifornia, comprises a thick volcaniclastic sequence deposited in a submarin e island-are setting and penetrated by numerous related hypabyssal intrusio ns. A composite andesite-diorite intrusive complex greater than or equal to 4.5 km long and greater than or equal to 1.5 km thick was emplaced while t he host Tuttle Lake sediments were still wet and unconsolidated. Large part s of the intrusive complex consist of peperite formed where andesitic magma intruded and intermixed with tuff, lapilli-tuff and tuff-breccia. The sout hern half of the complex consists of augite-phyric andesite containing pepe rite in numerous small, isolated pockets and in more extensive, laterally c ontinuous zones. The peperites comprise three main types recognized previou sly in other peperite studies. Fluidal peperite consists of small (less tha n or equal to 30 cm), closely spaced, at least partly interconnected, globu lar to amoeboid andesite bodies enclosed by tuff. This peperite type develo ped during intrusion of magma into fine-grained wet sediment along unstable interfaces, and fluidization of the sediment facilitated development of co mplex intrusive geometries. Blocky peperite and mixed blocky and fluidal pe perite formed where magma intruded coarser sediment and underwent variable degrees of brittle fragmentation by quenching and dynamic stressing of rigi d margins, possibly aided by small steam explosions. The northern half of t he intrusive complex consists predominantly of a different type of peperite , in which decimetre-scale plagioclase-phyric andesite clasts with ellipsoi dal, elongate, or angular, polyhedral shapes are closely packed to widely d ispersed within disrupted host sediment. Textural features suggest the ande site clasts were derived from conduits through which magma was flowing, and preserved remnants of the conduits are represented by elongate, sinuous bo dies up to 30 m or more in length. Disruption and dispersal of the andesite clasts are inferred to have occurred at least partly by steam explosions t hat ripped apart a network of interconnected feeder conduits penetrating th e host sediments. Closely packed peperite is present adjacent to mappable i ntrusions of coherent andesite, and along the margin of a large mass of coa rse-grained diorite. These coherent intrusions are considered to be major f eeders for this part of the complex. Examples of magma/wet sediment interac tion similar in scale to the extensive peperites described here occur elsew here in ancient island-are strata in the northern Sierra Nevada. Based on t hese and other published examples, large-scale peperites probably are more common than generally realized and are likely to be important in settings w here thick sediment sequences accumulate during active volcanism. Careful m apping in well-exposed terrains may be required to recognize large-scale pe perite complexes of this type.