Question 1 of 7: Carbonates — Genesis and Two Contrasting Types

Question 1: Carbonates — Genesis and Two Contrasting Types (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Carbonate genesis: allochemical versus orthochemical

Carbonate rocks are overwhelmingly of biochemical and biogenic origin, precipitated directly or indirectly by organisms in warm, shallow, clear, sunlit marine water (the "carbonate factory"), which is why carbonate deposition is largely restricted to low-latitude, siliciclastic-starved shelves and platforms. Folk's classification splits carbonate constituents into two fundamentally different genetic classes that map onto the "two types" the question asks for: allochemical (clastic) carbonates, built from discrete carbonate particles — allochems — that were produced, then transported and reworked by currents and waves before final deposition, exactly analogous to a siliciclastic sandstone; and orthochemical (non-clastic, chemical) carbonates, precipitated essentially in place with no significant transport, either directly from seawater (evaporites, some micrites, cements) or by in-place organic construction (reef framework, stromatolitic lamination).

Clastic (allochemical) carbonates

Physical: allochems — ooids, peloids, intraclasts and bioclasts (skeletal fragments of mollusc valves, echinoderm plates, foraminiferal tests, coral fragments) — are sorted, rounded and transported by waves and currents just like quartz sand, so allochemical limestones display the same primary sedimentary structures as siliciclastic sandstones: cross-bedding, ripple lamination, grading and channel scour. Texture (grain size, sorting, rounding) and the amount of interstitial lime mud (micrite) matrix versus sparry calcite cement record the depositional energy exactly as in a sandstone (high energy winnows mud and leaves a grain-supported, cement-bound grainstone; low energy leaves a mud-supported wackestone/packstone). Chemical: mineralogically the grains are aragonite or high-Mg calcite as precipitated by the source organism, which is metastable and converts to low-Mg calcite (or, with Mg and evaporitic brine involvement, dolomite) during early diagenesis — a chemical instability that has no siliciclastic equivalent and drives extensive dissolution, cementation and neomorphic recrystallization even before burial. Engineering: the depositional fabric (grain-supported vs mud-supported) and the degree of early cementation directly set intergranular porosity/permeability and unconfined compressive strength, so allochemical carbonate reservoirs and foundation units are exploration/design targets in exactly the way a clastic sandstone body is — mapped as a facies belt with predictable lateral continuity and quality trends away from the high-energy shoal axis.

Non-clastic (orthochemical/chemical) carbonates

Physical: in-place precipitates and framework builds show no current-transport structures at all; instead they display growth fabrics — fenestral (birdseye) porosity in tidal-flat micrites, laminated stromatolitic/thrombolitic fabric built by microbial mat trapping and binding or direct precipitation, and rigid, wave-resistant reef framework built by corals, calcareous algae and other frame-builders that bind sediment in place. Chemical: precipitation is controlled directly by seawater carbonate saturation state, temperature, salinity and CO2 degassing/photosynthetic drawdown (the equilibrium CaCO3 + CO2 + H2O ⇌ Ca2+ + 2HCO3- shifts toward precipitation when CO2 is removed by warming, agitation or algal photosynthesis, which is why ooid shoals and stromatolites form in warm, agitated or microbially-influenced shallow water); evaporitic orthochemical carbonates (and associated dolomite) additionally record progressive concentration of seawater past gypsum/anhydrite saturation. Engineering: reef and mound frameworks are rigid, wave-resistant, high-primary-porosity bodies with abrupt facies boundaries (a reef core can sit directly against open-shelf mud with almost no gradational zone), so they form geometrically discontinuous, hard-to-predict engineering units — excellent local foundation rock or hydrocarbon traps, but a poor guide to conditions a short lateral distance away; tidal-flat and evaporitic orthochemical carbonates are commonly thinly interbedded with anhydrite/gypsum and are prone to dissolution (karst, sinkholes) long after burial, a major geotechnical hazard for foundations and dam sites.