112 SEDIMENTARY ROCKS/Limestones to deposit thick formations of peloidal packstones and wackestones In arid climates, these sediments may, in turn, pass into ‘sabkha’ (‘sabkha’ is Arabic for salt marsh) where dolomite and evaporite minerals may form In humid climates, where terrigenous sediment runs off from the land, the carbonate lagoons may interfinger with siliciclastic sand and mud The depositional environments of carbonate shorelines and shelves in general, and reefs in particular, are described in greater detail in (see Sedimentary Environments: Carbonate Shorelines and Shelves; Reefs (‘Build-Ups’)), respectively Limestone Diagenesis As noted earlier, the minerals that form limestones are far less stable in the subsurface than are those that form sandstones Recent carbonate sediment at the Earth’s surface is composed of the two isomorphs of calcium carbonate: aragonite and calcite Recent lime mud is largely aragonitic, but skeletal material is composed of both varieties, which vary in importance between different animal and plant groups The change of unconsolidated lime sediment into limestone happens very quickly, and with negligible burial The ‘fossilized’ beer bottles and other anthropogenic detritus found in modern ‘beach rock’ prove this These early cements are of both calcite and aragonite In skeletal sands, one of the first diagenetic reactions is the dissolution of aragonite shells This generates biomoldic porosity During burial, aragonitic muds undergo a reordering of the crystal lattice to form calcite This change is concomitant with a volumetric increase of 8%, and a corresponding loss of porosity This is why most ancient lime mudstones, certainly those of pre-Mesozoic age, are normally hard, tight, splintery rocks By contrast, many Cretaceous and younger lime mudstones are light, porous, and chalky Chalk consists mainly of the fossils of planktonic algae, termed the Coccolithophoridae, together with their disaggregated skeletal plates, termed coccoliths, coccolith-rich faecal pellets, calcispheres, and unicellular planktonic foraminifers Coccoliths are not composed of unstable aragonite, but of the stabler calcite Thus, during burial, these lime muds not undergo expansive diagenesis like aragonitic muds They maintain their chalky texture, being highly porous, but normally impermeable unless fractured Chalks are described in greater detail in (see Sedimentary Rocks: Chalk) Returning to the diagenesis of carbonate sands, during shallow burial, early cementation may destroy some porosity, but aragonite dissolution may enhance it With continued burial, calcite cement may infill both biomolds and any remaining intergranular porosity There are, however, several other diagenetic processes to which a cemented limestone may be subjected Limestones may undergo recrystallization, during which some or all of the primary fabric may be destroyed Individual carbonate grains, generally bioclasts or ooids, may undergo pressure solution This is a process whereby dissolution occurs at grain contacts due to overburden pressure Concomitantly, the dissolved mineral matter may be precipitated as cement in adjacent pores Additional evidence of dissolution is provided by stylolites These are sutured surfaces, generally subparallel to bedding, where extensive dissolution has left an insoluble residue of clay, kerogen, and other matter along the suture Stylolites occur in both pure limestones and quartzose sandstones Limestone diagenesis must not be thought of as a ‘one-way street’ that leads to the total loss of porosity and permeability Limestones may be flushed through with acidic pore fluids, whose leaching properties may generate secondary porosity and permeability The acidic fluids may come from adjacent compacting clay beds, conveniently generating secondary porosity ahead of petroleum invasion More usually, however, secondary solution porosity is the result of uplift and erosion, and the flushing of limestone by acidic meteoric water (there is nothing new in acid rain) Solution may form moldic and vuggy pores It may enlarge fractures and, in extreme cases, develop karstic caverns with concomitant collapse breccias (see Sedimentary Processes: Karst and Palaeokarst) Many of the best carbonate petroleum reservoirs occur where solution porosity has been developed and preserved beneath unconformities The best way of preserving porosity in a limestone is for petroleum invasion to occur and expel cementing connate fluids Renewed burial, without the benefit of petroleum invasion, may, of course, result in total recementation of the limestone as it makes its way to a completely cemented and recrystallized rock, termed marble The last important diagenetic process to which limestones are subjected is dolomitization, a process of such complexity and importance that it merits an article to itself (see Sedimentary Rocks: Dolomites) Economic Importance of Limestones Limestones are of great economic importance for many reasons First, limestones contain lime, an essential ingredient for plant growth, and so limestone quarries are ubiquitous adjacent to farmland with lime-poor acid soil Hard cemented limestones make excellent building stone and aggregate Porous and permeable limestones, by contrast, serve as aquifers