how does sediment compact or cement into sedimentary rocks?
Lithostatic load! As sediment is deposited into basins, the pressure from the weight of the overlying sediment presses and squeezes the seds into hard, rocky bodies. Water plays a part of this as well. Water can allow for the precipitation of cement in sandstone (Calcium Carbonate).
Just speaking to sediments in general. Sediments become sedimentary rocks by one of two mechanisms -- cementation and compression. Cements “glue” one particle to another and commonly include: (1) calcite (CaCO3) (2) silica (SiO2) (3) limonite (Fe2O3) (4) hematite (Fe2O3.xH2O) Water flowing through certain sediments becomes supersaturated with calcite or silica. When this water evaporates, these dissolved minerals precipitate around the grains through which they flow. Over time, the precipitation of these cements overlaps, binding one grain to another. But the case of hematite and limonite cements is a little different. Iron occurs in solution in its ferrous (Fe+2) state. When ferrous iron encounters oxygen, it oxidizes into its ferric (Fe+3) state, which is insoluble. This ferric iron precipitates, coating the grains through which it flows with either hematite or limonite. It coats the grains, eventually binding one grain to the next. Sedimentary rocks commonly cemented include conglomerates, sandstones, evaporates, ironstones, as well as some limestones, some cherts, and some phosphorites. This process of cementation involves heat that allowed these fluids to exist and flow, but not so much as to cause a metamorphic rock to be created. Compression involves finer grained sediments. Clay, organic matter, lime mud, and phosphorite mud, once buried, are subjected to progressively more overburden. The thickness of these layers is significantly reduced as water is “squeezed” out. These fine particles have very high surface areas and surface charges causing these grains attach to one another. In addition, surface irregularities become intertwined, holding grains together. Sedimentary rocks that are formed from sediments by compression include shale and coal as well as some limestones, some cherts, and some phosphorites.
