Page 36 - Carbonate Facies in Geologic History

Compositional Terms 23

Compositional Terms
(Often employed both descriptively and with genetic implications.)
Sediment pile: An accumulation of sediment in almost any shape whose inter-
nal composition shows it to originate mainly through mechanical piling of moved
particles (e.g., dunes, bars, spits, tidal deltas, etc.). The term bank has also been
used to mean a sediment pile.
Organic bank: Buildup whose internal composition permits the inference that
it is formed mostly of detrital organic sediment accumulated in place by trapping
or baffiing but also in part through mechanical piling by waves and currents.
Bioherm: Buildup whose internal composition shows it to be largely derived
from in situ production of organisms or as framework or encrusting growth as
opposed to mainly mechanical (hydrodynamical) piling.
Lime mud mounds and linear mud accumulations: Lime mud matrix dominates
other constituents such as organic boundstone and bioclastic debris. Such build-
ups are commonly perceived to accumulate both through hydrodynamic pro-
cesses and in situ organic production.
Organic framework reef or ecologic reef (Dunham, 1970): Buildup formed in
part by a wave-resistant framework constructed by organisms. An accessory part
of such a definition implies that the reef exerts some degree of control over its
surrounding environments. Other types of buildups behave similarly. Some geol-
ogists have more or less informally used the simple term "reef' for this concept.
Others, principally petroleum geologists, have advisedly used "reef' alone for any
carbonate buildup. The original term meant a ridge or shoal on which a ship
could ground. Because many such features were coral-algal reef, geologists appro-
priated the term to their own special jargon and with two specialized meanings:
(1) organic framebuilding communities and (2) organic buildUps. Many earth sci-
entists wish that the simple term "reef' could be returned to navigators and
dropped altogether from geological use. It is undoubtedly too late for this. An
obvious solution is to modify the term whenever it is used.
Heckel (1974), in an attempt to make compositional terms more objective,
proposed the terms in Table II-1 in a long essay on carbonate buildups. Though

Table II-I. Terminology for carbonate buildups
Heckel (1974) Common usage followed in this text; Dunham
Compositional descriptive descriptive with genetic implications (1970)
terminology

Major Encrusted Organic Bioherm Carbonate Ecologic
mixed skeletal buildup framework reef buildup reef
buildups
Loose skeletal Organic bank Strati-
buildup graphic reef
Lime mud Lime mud
buildup accumulation
Sorted-abraded Sediment pile
skeletal buildup