Early subsea cement and cemented sediment are restricted to marginal facies of shallow Belize barrier and atoll reefs. In these facies cementation is not ubiquitous, but is sporadic within seaward-facing spurs and within the upper meter of the reef pavement. The presence of internal sediment, composed of well-sorted lime silt, is critical to the precipitation of rock-making cement. This lime silt is made up of Mg-calcite peloids of unknown origin and angular chips of aragonite excavated from coral and mollusk skeletons by boring sponges. In Belize the reef flat pavement, a subtidal bed of lithified coral conglomerate 1 m thick and 10 to 100 m wide, occurs in the lee of all marginal reefs. Acropora palmata fronds and logs (radiocarbon age ca. 450 years) are cemented together by a mortar of lime packstone to wackestone. In seaward-facing spurs, cavities between in situ coral and Millepora are locally filled with cemented skeletal packstone. Cemented packstone and wackestone also partially fill intraskeletal pores and borings in coral fronds lying loose on the sea floor. The predominant cement of internal sediments is Mg-calcite (14.5-18.6 mole % Mg) as micrite and bladed spar. Aragonite, although common, is found only in coral pores. The crystal form of the cement and the degree of lithification can be related to variations in the texture of the internal sediments. Mg-calcite micrite, the first cement, develops preferentially in the small pores, less than 15 mu , between lime silt grains. Mixed lime sand and lime silts, with a wider range of pore sizes, are cemented after the lime silts by thin rinds of micrite that grade outward into bladed spar. In the larger pores of grainstones, bladed spar follows thorough cementation of lime silts in the same sample.