Geology Reference
In-Depth Information
Fig. 7.13.
Diagenesis across the Bahama Banks
(modified after Demicco and Hardie 1994) demonstrated by the distribu-
tional patterns of diagenetic processes, products, and early diagenetic cement types over the shallow platform and in adjacent
deep-marine slope and basinal settings. Inter- and intragranular cements occur in tidal flat and shelf lagoon peloid muds, in
bank-margin sands, in bank-margin reefs, in foreslope and periplatform debris. These cementation sites have several criteria
in common: sedimentation and erosion rates are low, host sediments have the same mineralogy as cements (aragonite and
High-Mg calcite) and can thus act as seeds, and large volumes of supersaturated waters are pumped through the sediment.
However, the specific conditions that promote the precipitation of CaCO
3
are different in each of these environments. Partial
lithification of tidal flat, lagoonal and sand shoal sediments produces hard surface crusts, hardgrounds, and hardened grains.
Intragranular cementation in fecal pellets and sand-sized mud aggregates turn soft muds of the outer lagoon deposited under
low-energy conditions into hard sand grains with significantly different hydrodynamic properties. In contrast to the tidal and
lagoonal sediments, the framework and debris of bank-margin reefs are extensively cemented due to the immense volumes of
seawater flushed by tides and waves through the reef, thus providing the prerequisite for the growth of pore-lining cements.
vironment as exemplified by the diagenesis across the
Bahama Banks (Demicco and Hardie 1994; Fig. 7.13).
Cementation by calcite or aragonite precipitation is co-
eval with sedimentation.
Early marine cementation occurring in shallow-ma-
rine plaforms and ramps at or near the sediment-sea-
water interface depends on several prerequisites:
• Very low sedimentation and erosion rates. Low sedi-
mentation rates and temporal non-deposition connected
with increased biogenic encrustations (e.g. in deeper-
marine environments) will enhance the possibility of
submarine cementation (formation of hardgrounds).
• Large volumes of sea water are pumped through the
sediments. Early cementation depends on sufficient car-
bonate. This is supplied by transport through the sedi-
ment by waves, tides, currents, or evaporation pro-
cesses. Strong cementation, therefore, is common at
seaward/windward sides of tidal and shallow subtidal
areas (e.g. carbonate sand shoals), but less frequent on
the protected leeward part of shallow-marine environ-
ments.
•
through the sediment and a sufficient supply of cement-
ing carbonate.
• Appropriate water temperatures and salinities.
• Biogenic activities producing pores (e.g. borings and
burrows) and stabilizing substrates (e.g. biofilms, al-
gal mats, sea grass areas) and preventing reworking of
carbonate grains. Sponge borings, e.g. may play a criti-
cal role in the aragonite-to-calcite inversion process of
shells (Rehman et al. 1994) providing conduits for me-
teoric alteration.
• Metabolic activity (photosynthesis and respiration)
as well as disintegration of organic material causing
dissolution of carbonate and controlling pCO
2
and the
degree of supersaturation (Jahnke et al. 1994).
7.4.4.2 Reefs
The most significant amount of cementation and there-
fore porosity reduction, occurs in reef environments
with higher rates of agitation and/or lower sedimenta-
tion rates (Lighty 1985). Common patterns are (a) high
Permeability, determining the flow of the sea water
