About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 63 (1979)

Issue: 3. (March)

First Page: 552

Last Page: 552

Title: Braided-Stream Processes and Facies in Modern Arc-Trench Gap, Southwestern Guatemala: ABSTRACT

Author(s): David L. White, W. David Kuenzi, R. V. McGehee

Article Type: Meeting abstract


Modern fluvial processes and facies in arc-trench gaps have been little studied even though prograding coastal-plain sequences may be the dominant facies in parts of ancient arc-trench gaps underlain by continental crust. The Pacific coastal plain in Guatemala is abruptly terminated 25 to 60 km inland from the Pacific shoreline by the steep slopes of an active Quaternary volcanic arc. The volcanic slopes are locally bare of vegetation and, in response to strongly seasonal torrential rainfall, provide abundant bed load to high-gradient, low-sinuosity braided streams that discharge onto the coastal plain and flow, in a roughly parallel pattern, into the Pacific.

Studies of the Rio Samala reveal systematic downstream changes in cross-channel bed-relief index (from 8.2 to 1.3), maximum particle size (from 180 cm to 6.4 cm), and facies composition. The dominant facies, in a tract from proximal to distal, are: proximal (0 to 17 km)--crudely bedded gravel that records flood-event processes associated with longitudinal-bar and boulder-string formation, as well as channel scour and plugging; mid-distal (17 to 40 km)--horizontally stratified sand deposited by flat-bed accretion in channels and on bars; and distal (40 to 53 km)--trough and tabular cross-stratified sand produced by migration of dunes and foreset bars. These facies characterize, respectively, the Scott-, Bijou-, and Platte-type braided-stream vertical sequences described by Miall.

Recognition of similar downslope changes in braided-stream sequences deposited in ancient arc-trench gaps should enhance interpretation of proximity to source, direction of paleoslope, shoreline trend, and trench orientation.

End_of_Article - Last_Page 552------------

Copyright 1997 American Association of Petroleum Geologists