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A Sedimentologic Perspective on Stratal Disruption within Sandstone-Rich Melange Terranes

Michael B. Underwood
The Journal of Geology
Vol. 92, No. 4 (Jul., 1984), pp. 369-385
Stable URL: http://www.jstor.org/stable/30062277
Page Count: 17
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
A Sedimentologic Perspective on Stratal Disruption within Sandstone-Rich Melange Terranes
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Abstract

There are two main schools of thought pertaining to the origin of sandstone-rich melange terranes such as the Franciscan Coastal Belt of northern California. One view maintains that the deformation is tectonic, associated with offscraping or underplating of detritus within a subduction zone. The other opinion is that early-phase disruption is caused by gravitational failure of trench-slope sediment. Sedimentary facies data from modern subduction zones have significant bearing on both of these interpretations. Such data, especially from regions bordering continental arcs, demonstrate that sandy deposits are concentrated within specific environments, most notably the wedge of turbidites common to trench floors; sands may also occur on the adjacent abyssal plain and within trench slope basins. In contrast, a typical apron of slope sediments consists almost exclusively of hemipelagic muds. Even though submarine slides are no doubt common on steep trench slopes, most such slides simply involve remobilization of the muddy slope cover. Facies associations within the Franciscan Coastal Belt contrast markedly with the muddy character of the slope apron; thus, it is unlikely that either deposition or subsequent gravitational failure occurred within such a setting. Instead, the sandy strata probably represent accreted and tectonically-deformed trench-floor or abyssal-plain deposits.

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