Siliciclastic grain breakage and displacement due to carbonate crystal growth: an example from the Lueders Formation (Permian) of north-central Texas, U.S.A.

The Lueders Formation (mid-Permian) in Baylor County, Texas, is an intercalated suite of fluvial siliciclastic, shallow marine siliciclastic, and shallow marine carbonate strata. There are at least two generations of carbonate cements (probably originally composed of calcite) in the fluvial sandstones where fractured grains are observed. These cements represent the initial stages of caliche formation. Cementation is envisioned as a two step process. In the first step, calcite cements form from supersaturated fluids in a freshwater, vadose environment as a meniscus cement at grain contacts. Areas of cement formation are restricted to these sites because fluid distribution is restricted to these sites. Stresses generated by the growth of cements at grain contacts are transmitted through and concentrated at quartz/quartz grain point-contacts until the stress is sufficient to fracture quartz grains, even though the ultimate strength of calcite is less than that of quartz, per unit area. This process occurs too rapidly to be accommodated by pressure solution. In the second phase of cementation, cement nucleation is no longer restricted by vadose conditions. In this phase, calcite growth can no longer result in quartz grain breakage; rather, the quartz grains are dispersed in poikilotopic calcite cement.