Abrupt Lateral Facies Research Articles

Alisitos Terrane, Baja California: Sedimentation Within Early Cretaceous Island Arc: ABSTRACT

The Alisitos terrane is a thick accumulation of mainly volcaniclastic sediments that were deposited in an island arc setting related to late Mesozoic subduction along the southwest margin of North America. A diverse suite of depositional facies can be related to various environments of a marine volcanic arc. The recognition of facies associations and thermal histories characteristic of specific paleoenvironments within the Alisitos terrane provides a model which to some extent is applicable to arc terranes in general. Important features of sedimentation in an arc setting include discrete point sediment sources, high sedimentation rates, and the maintenance of steep slopes, and abrupt lateral facies changes. Development of shallow-water environments within the Alisitos terrane was related to major volcanic centers. Characteristic facies include coarse proximal volcaniclastic rocks, locally interspersed with rudist limestone bodies, and relatively abundant hypabyssal rocks and associated thermal alteration. Sediment was dispersed into deep water by both epiclastic and pyroclastic processes. Epiclastic transport was dominantly by mass-flow processes, removing coarse volcanic sediment from the flanks of volcanic edifices to intra-arc and fore-arc basins. Sediments also reached these sites by pyroclastic processes. Criteria for distinguishing deposits of pyroclastic and epiclastic origin in these basins include recognition of Bouma End_Page 897------------------------------ sequences (graded bedding characterizes both facies), and single versus multiple clast types. End_of_Article - Last_Page 898------------