Geology Reference
In-Depth Information
Substrate types of cold-water carbonates
Carbonate sands formed in temperate and cold-wa-
ters of shelf platforms are characterized by the predomi-
nance of calcitic skeletal grains that are permanently
reworked (Sect. 2.4.4.3).
Cements are usually restricted to intraskeletal pores.
Interskeletal cementation is weak due to the scarceness
of primary aragonitic grains that could provide carbon-
ate for cementation after dissolution. Sands remain
unlithified for long periods, therefore many substrates
are loosegrounds or firmgrounds.
Box 12.5.
Criteria for recognizing carbonate substrate
types
(for trace fossils see Goldring 1991, 1995; Bromley
1996).
Biota
•
Distribution of deposit feeders and suspension feed-
ers.
•
The ratio of epifauna and endofauna provides infor-
mation on substrate stability. The mode of feeding is
controlled by grain size: Fine-grained soft substrates
are dominated by endobenthos; increase of substrate
stability with increasing grain size results in an in-
crease in epibiontic elements, deposit feeders decrease
in coarse-grained sediments due to the early decom-
position of organic matter.
Basics: Carbonate substrates
Bottjer, D.J., Ausich, W.I. (1986): Phanerozoic development
of tiering in soft substrate suspension-feeding communi-
ties. - Paleobiology,
12
, 400-420
Brett, C.E. (1988): Paleoecology and evolution of marine hard
substrate communities: an overview. - Palaios,
3
, 374-378
Copper, P. (1992): Organisms and carbonate substrates in
marine environments. - Geoscience Canada,
19
, 97-112
Ekdale, A.A. (1985): Paleoecology of the marine endo-
benthos. - Palaeogeography, Palaeoclimatology, Palaeo-
ecology,
50
, 63-81
Fabricius, F. (1968): Calcareous sea bottoms of the Rhaetian
and Lower Jurassic sea from the west part of the Northern
Calcareous Alps. - In: Müller, G., Friedman, G. (eds.):
Recent developments in carbonate sedimentology in cen-
tral Europe. - 240-249, Berlin (Springer)
Feldman, H.R., Brown, M.A., Archer, A.W. (1993): Benthic
assemblages as indicators of sediment stability: evidence
from grainstones of the Harrodsburg and Salem limestones
(Mississippian, Indiana). - In: Keith, B.D., Zuppann, C.W.
(eds.): Mississippian oolites and modern analogs. - Ameri-
can Association of Petroleum Geologists, Studies in Ge-
ology,
35
, 115-128
Goldring, R. (1995): Organisms and the substrate response
and effect. - In: Bosence, D.W.J., Allison, P.A. (eds.):
Marine paleoenvironmental analysis from fossils. - Geo-
logical Society, London, Special Publications,
83
, 151-
180
Purdy, E.G. (1964): Sediments as substrates. - In: Imbrie, J.,
Newell, N.D. (eds.): Approaches to paleoecology. - 238-
271, New York (Wiley)
Further reading
: K172
•
Mode of life (e.g. cemented versus burrowing or bor-
ing; provides information on the consistency of the
substrate).
Trace fossils
•
Burrowing, bioturbation and ichnofabric pointing to
soft bottoms (Sect. 5.1.4; Fig. 5.4; Pl. 103). Burrows
are formed in muds, sands and firmgrounds (Pl. 19/
1, 3, 7).
•
Stages in the preservation of trace fossils provide in-
formation of different stages of hardening.
•
Tiering. Animals producing trace fossils live at dif-
ferent levels (tiers) in the sediment and colonize newly
deposited sediment at different rate in different ways.
Tiering patterns offer information on substrate con-
ditions, sedimentation rates and oxygenation levels
(Bromley and Ekdale 1986; Wetzel 1991; Uchman
1995).
•
Abundant macro- and microborings on bed surfaces
pointing to hardgrounds and firmgrounds (Sect. 9.3).
Microfacies criteria
•
Depositional texture types (lime mudstones versus
grainstones).
•
Biofabrics and current orientation of grains (Sect.
5.1.2) pointing to looseground conditions.
•
Biolamination point to firmground conditions (Sect.
5.1.3).
•
Mineralized crusts on bed surfaces point to hard-
grounds (Sect. 5.2.4.1 and Sect. 9.2; Pl. 23/2, 3, 7).
•
Disturbances of sedimentary fabrics and structures.
•
Differences in grain size, sorting and packing.
12.1.4 Light
Large-scale reductions in the incident light of benthic
calcareous red algae have been suggested as causes for
the mass extinction at the end of the Cretaceous (Aguirre
et al. 2000), The functioning of the oceanic ecosystem
is dependent for energy to a high degree on the photo-
synthetic activity of phytoplankton confined to a thin
zone of lighted surface water. Light-dependent photo-
synthesis is also of fundamental importance for marine
benthic plants (algae and sea grass), and for the life of
many benthic invertebrates harboring photosynthetic
endosymbionts.
Bioturbation and substrate types
Because fine-grained sediment contains more or-
ganic matter than coarse-grained sediment, burrowing
and bioturbation fabrics are common in micritic lime-
stones (Sect. 5.1.4). The differentiation of soft-, loose-
and firmgrounds is primarily based on the preservation
of burrows (Goldring 1995). An example is shown in
Pl. 103.
