ELSEVIER Sedimentary Geology 110 (1997) 277-297 Sedimentary Geology Holocene cemented beach deposits in Belize • a* Eberhard Glschler ', Anthony J. Lomando b a institutj~ur Geologie und Palaontologie, Universitiit Tiibingen, Sigwartstrasse 10, 72076 Tiibingen, Germany b Chevron Overseas Petroleum Inc., 6001 Bollinger Canyon Road, San Ramon, CA 94583, USA Received 25 June 1996; accepted 12 November 1996 Abstract Two types of cemented beach deposits occur on reef islands off the coast of Belize. These are (1) intertidal beachrock that is dominantly cemented by marine aragonite and high-magnesium-calcite cements, and (2) supratidal cayrock that is cemented mainly by vadose low-magnesium-calcite cements. Besides differences in position relative to present sea level and resulting eaxly diagenetic features, beachrock and cayrock can be distinguished on the basis of differences in composition, texture, geographical position, and age. Whereas the composition of beachrock is similar to that of the adjacent marginal reef sediments, cayrock is enriched in benthic foraminifera. Intertidal beachrock is moderately to well sorted and well cemented, while supratidal cayrock is very well sorted, poorly cemented and friable. Beachrock occurs preferentially on windward beaches of sand-shingle cays on the middle and southern barrier reefs and on the isolated platforms Glovers and Lighthouse Reefs. Cayrock only occurs on larger mangrove-sand cays of the isolated platforms Turneffe Islands, Lighthouse Reef, and the northern barrier reef. Lac-dating of ten whole-rock and mollusk shell samples produced calibrated dates between AD 345 and AD 1435 for beachrock and between BC 1085 and AD 1190 for cayrock. The large-scale ctistribution of beachrock in Belize supports the contention that physical processes such as water agitation rather than biological processes control beachrock formation and distribution. Only on windward sides of cays that are close to the reef crest, where large amounts of seawater flush the beaches, considerable amounts of cements can be precipitated to produce beachrock. Cayrock forms due to cementation in the vadose zone and is only preserved on larger, stable mangrove-sand cays. Keywords: beachrock; C-14; carbonate rocks; cementation; diagenesis; Belize 1. Introduction The passive con~inental margin of Belize is struc- tured by a series of five NNE-striking fault-blocks that form the basement of the 250 km long bar- rier reef and the l~tree isolated platforms Glovers Reef, Lighthouse Reef, and Turneffe Islands (Fig. 1). Recent tectonic movements along the block-faults *Corresponding author. Fax: +49 7071-949040. E-mail: eber hard.gischler @ uni-tuebi~ngen.de which are probably caused by activity at the nearby boundary between American and Caribbean plates are indicated by surface-breaking faults in seismic profiles through the continental margin (Dillon and Vedder, 1973) and tilted Pleistocene stalactites in the Blue Hole on Lighthouse Reef (Dill, 1977). Further indications are frequent earthquakes in Honduras and recent earthquake activity, greater than magni- tude 4.0, recorded on Glovers Reef, west of Turneffe Islands, and along the central part of the barrier reef in Belize. 0037-0738/97/$17.00 4.) 1997 Elsevier Science B.V. All fights reserved. PIIS0037-0738(96)00088-7