580 SEDIMENTARY ENVIRONMENTS/Storms and Storm Deposits Storms and Storm Deposits P Myrow, Colorado College, Colorado Springs, CO, USA ß 2005, Elsevier Ltd All Rights Reserved Introduction Numerous studies of modern shelves indicate that storms have wide-ranging effects in a spectrum of environments from outer shelves to coastal plains Early oceanographic studies led to interest in finding analogues in the rock record Resulting facies models for storm-influenced depositional systems were not published until the late 1970s and early 1980s, later than for many other depositional systems (e.g fluvial, tidalinfluenced shoreline, submarine fan) There are a number of reasons for this First, the oceanographical study of modern storms is extremely difficult Direct observations are challenging and dangerous Quantitative analysis also required the development of current meters and other devices that could survive the effects of strong storm-generated currents and waves Second, storms are extremely complex systems that include strong currents and waves, both of which vary both temporally and spatially during an individual storm Third, although tempestites, individual storm-generated sandstone beds, were attributed to deposition from storm-generated currents fairly regularly by the mid-1970s, features considered to be diagnostic of deposition by storm processes, such as hummocky crossstratification (HCS), were not identified early in the study of sedimentary structures Many case studies of ancient storm-influenced deposits and modern settings indicate that storms have wide-ranging effects in a spectrum of environments from outer shelves to coastal plains The most common storm deposits in the rock record are interbedded sandstone and bioturbated shale (Figure 1) The sandstone beds were deposited as a result of powerful near-bottom water motions produced by waves and currents, and these are the main focus of the following discussion Additional storm-generated features include gutter casts, pot casts, gravel and conglomerate accumulations, and reworked or transported shell beds Early Facies Models Early studies established the basic idea that storms result in the transport of sediment from generally well-sorted shoreline sources onto shelves, as deduced from lithofacies patterns of decreasing bed thickness and evidence for less powerful currents from onshore to offshore Despite more than 30 years of work on storm-influenced shorelines and ancient deposits, there remains considerable debate concerning the processes by which sediment is transported seawards during storms Early discussions of storm processes produced two radically different perspectives from oceanographers and from geologists who worked on ancient deposits Stratigraphical and sedimentological data indicated a potential role of density-induced forces, in part because many tempestites closely resemble turbidites The first facies model of storminfluenced shelves invoked the mobilization of sand at the shoreline, the creation of dense dispersions, and subsequent gravity-driven offshore transport Data from many ancient deposits indicate transport nearly perpendicular to the shore for long distances In this view, storm-deposited sandstone beds are indicative of nonuniformitarian processes, and the very rare storm events responsible for these beds (with recurrence intervals of tens of thousands of years) are presumably significantly different (quantitatively if not qualitatively) from those studied by oceanographers Oceanographic Studies Figure The deposits of storm influenced shelves consist of tempestites or storm generated sandstone beds and shale beds deposited between storm events Note how the sandstone beds tend to pinch and swell in thickness, a common feature of tem pestites These strata, exposed in the central Transantarctic mountains, were deposited along the East Antarctic continental margin during the Cambrian Hammer for scale Although the results of major storms on modern shorelines were first described in the late 1960s, the dynamics of modern storm-generated flows were not outlined until the 1980s by oceanographers who described the effects of storms on shelf and nearshore circulation Winds generated by storms move surface water, which is in turn deflected by Coriolis