Geology Reference
In-Depth Information
bounded above and below by distinct surfaces are called
bedsets
(Collinson and Thompson 1982).
Limestones consisting of thin, strictly parallel or
non-parallel laminae are named laminites (Sect. 5.1.3).
The descriptive term 'Plattenkalk' refers to sequences
composed of fine-grained, laminated limestones, which
are bedded in flat centimeter-decimeter thick, parallel-
sided units (Swinburne and Hemleben 1994).
Field studies of bedding and stratification must con-
sider (1) boundary planes and bedding surfaces, (2) bed
thickness, (3) composition and internal structure of
beds, and (4) vertical bed sequences.
(1)
Boundary planes and bedding surfaces
: Upper
and lower surfaces of beds are called
bedding planes
,
most of which are caused by abrupt changes in deposi-
tional conditions, non-deposition or erosion. However,
bedding planes also result from diagenetic processes
or weathering. Pressure solution and stylolite seams par-
allel to depositional bedding may produce 'pseudo-bed-
ding' simulating true depositional bedding, and lead-
ing to sampling problems, because bed-to-bed sampling
usually relies on the assumption of sedimentary bound-
ary planes.
Bedding surfaces are abrupt planar, abrupt irregular
or gradational, with even (flat), wavy or curved sur-
faces that are parallel or non-parallel to each other. Dif-
ferences in bedding surfaces provide information re-
lated to the time span of non-deposition (e.g. scalloped
or irregular potholed surfaces, sometimes with paleo-
sols, indicate a paleokarst stage; irregularly shaped hard-
grounds may point to deep-marine solution).
Sedimentary structures on top and bottom surfaces
of bedding planes provide information about deposi-
tional energy and substrate features. Some of the more
common structures on the top surfaces of limestones
are modifications of surface topography, bed forms
(ripples, dunes, hummocks), shrinkage cracks and pri-
mary current lineations, and various trace fossils. Struc-
tures seen on bottom surfaces include flute marks,
groove marks, tool marks, scour structures as well as
load marks.
Fig. 3.3.
Geometries of carbonate rock layers.
Thin sections.
Left -
Planar fine-laminated
limestone characterized by
alternating millimeter-scale lamina couplets consisting of
mudstone (dark gray) and packstone (light streaks). The black
line in the center and the black spots near the top are bitumina.
Note the extremely fine lamination within the lamina couplets,
the parallel orientation of fine shells and the concave-up
position of the ostracod shell near the top of the sample. These
criteria indicate suspension settle-out of mud (calcisiltite)
within a quiet-water environment, alternating with sporadic
influx of peloids, clay and terrigenous quartz grains transported
by gentle traction currents.
The sample represents an organic-rich facies formed in
small isolated, tectonically induced basins within an intertidal
to subtidal carbonate platform. The bitumen-rich limestones
were deposited from a stratified water body with anaerobic
and dysaerobic conditions (Czurda 1973; Hopf et al. 2001).
Large-scale cyclicity was controlled by sea-level change,
small-scale cyclicity was controlled by climatic and seasonal
fluctuations. These limestones are regarded as potential
carbonatic source rocks.
Right -
Wavy-crinkled fine laminated
dolomite characterized
by irregularly undulated stromatolitic layers. Black laminae
are interpreted as the result of microbial activity (binding of
sediment, accumulation of organic material). The sample
represents the intertidal part of the Hauptdolomit platform.
Both samples are from the Late Triassic Hauptdolomit of
the Wiestal quarry, Gaissau, Salzburg/Austria. Scale is 5 mm.
Stratification
: A bed is a tabular or lenticular layer
that is sufficiently distinct from under- and overlaying
strata with respect to lithology, composition and tex-
ture and separated from these strata by recognizable
boundary planes. Genetically, a bed may represent a
depositional episode during which physical conditions
did not change significantly ('sedimentation unit': Otto
1938). Following the proposal of McKee and Weir
(1953) the term
bed
should be limited to strata thicker
than 1 cm. Very thin strata (thickness <1 cm) are called
laminae
(Campbell 1967). Laminae are the result of
changing depositional conditions causing variations in
the grain size, texture, mineral composition, and con-
tent of clay and organic material. Groups of laminae
forming a distinct sedimentary structure are called
laminasets
. Groups of similar beds or cross-beds,
(2)
Bed thickness:
Terminologies for the bed and
laminae thickness have been proposed by various au-
thors (McKee and Weir 1953; Ingram 1954; Reineck
and Singh 1980), but none has been generally accepted.
Ingram's scale (used by many authors) and the Shell
approach (distinguishing only 'ranges') are to be rec-
ommended (Fig. 3.2). Demicco and Hardie (1994) dis-
tinguish three layer categories based on layer thickness,
layer boundary geometry, and textural layer composi-
tion. This classification is a practical key to the identi-
