Geology Reference
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normal high tide during storm flooding. Diagnostic
criteria for supratidal environments are
Lamination should be studied with regard to the
(1) lithologic composition (only limestone or dolomite,
or limestone-dolomite couplets), (2) geometry of the
laminae (undulation; parallel laminae; cross-bedded
laminae), (3) extension of laminae (continuous, dis-
rupted), (4) microfacies of the laminae (micrite, pel-
micrite, pelsparite), (5) type and frequency of associ-
ated fenestral pores. See Read (1975) for a classifica-
tion of lamination types.
->Note: Parallel lamination occurs in subtidal, inter-
tidal and supratidal environments, but is especially
abundant in the intertidal zone.
Stratification: Bedforms and stratifications made by
waves and currents (ripples, planar and cross stratifi-
cation, winnowed sand lenses; flat and cross-bedded
calcarenites) exhibit a wide variety of types. Further
information can be found in Demicco and Hardie (1994).
-> Note: Common in the shallow subtidal and lower
intertidal zone.
evidence of subaerial exposure and pedogenic in-
fluence,
evidence of cementation in the vadose zone.
Intertidal
This zone is alternately flooded and exposed. Inter-
mittent exposure is indicated by desiccation features,
muddy surfaces with animal tracks and trails, and pores
filled with vadose cement. Characteristic features of
the intertidal regime are alternating erosion and depo-
sition and rapid changes in current and wave velocity.
These processes result in discontinuous sedimentation
associated with scour-and-fill and the formation of
channels, accumulation of reworked sediment (e.g. intra-
formational conglomerates), and substantial changes
in grain size from lamina to lamina and bed to bed.
Subtidal
This zone below the intertidal zone is permanently
submerged. Water depth is a few meters. The subtidal
zone is conventionally subdivided into a shallow and a
deeper subtidal zone.
The shallow subtidal zone is characterized by cal-
careous algae and invertebrates adapted to the phytal
(e.g. foraminifera, bryozoans, worms; see Voigt 1956).
Epiphytes are common. Important indicators are the
occurrence of dasyclad algae and encrusting organisms.
The lower boundary, after comparison with recent zo-
nations, can be drawn at approximately 30 m. The
deeper subtidal zone is characterized by high-diver-
sity benthic organisms. Calcareous algae still occur (red
algae). Dasyclads, however, are absent.
Desiccation features
Desiccation and shrinkage cracks seen on bedding
surfaces are considered as one of the most useful sedi-
mentary structures for identifying subaerial exposure
and ancient carbonate tidal flats. Desiccation cracks
formed by drying out of subaerially exposed mud have
different forms depending on the rate of drying, pres-
ence or absence of protecting microbial mats, expo-
sure time and bed thickness, and comprise three types:
Mud cracks (Pl. 25/5) and mud polygons.
Prism cracks: Polygonal network of cracks, essen-
tial normal to bedding, that breaks up the sediment into
vertical prisms.
Sheet cracks: Planar to gently undulatory cracks,
commonly parallel to the bedding and filled with sparry
calcite or micrite.
->Note: Desiccation mud polygons are common in the
supratidal and upper intertidal zone. Because mud
cracks can also form by substratal and subaqueous pro-
cesses, much care must be taken in interpreting mud
crack structures in ancient carbonates (see Demicco and
Hardie 1994).
15.5.1.2 Major Facies Criteria of Peritidal
Carbonates
Bedding
Lamination (Pl. 18/2; Pl. 123/1): A very common
feature of tidal carbonates is fine-scale lamination, usu-
ally on a millimeter scale (Sect. 5.1.3). Lamination of-
ten occurs associated with fenestral pores. The sedi-
ment may be trapped and bound by microbial mats,
giving rise to the alternation of fine-grained sediment
layers and irregular crinkled layers. Many ancient per-
itidal limestones exhibit a lamination consisting of mil-
limeter micrite or dolomicrite layers with thin organic
or bituminous seams, and thicker, up to several centi-
meters, layers composed of grainstone or packstone
with intraclasts and peloids. The latter represent storm
deposits.
Tepees
A special form of polygonal structures are tepees
(similar to inverted depressed Vs resembling Red In-
dian tents) characterized by antiformal structures with
sharp apices. Tepees originate from desiccation, but the
formation of large peritidal tepees is also controlled by
fluctuating groundwater conditions. They are associ-
ated with fractured and bedded tidal carbonates. The
fractures are filled with marine-vadose travertines and/
or terra rossa.
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