Geology Reference
In-Depth Information
Plate 102 Storm Deposits (Tempestites)
Limestone and sandstone beds deposited by storms are called tempestites. These storm deposits form beds and
lenses intercalated in marls and shales or in limestones of different texture and microfacies. Tempestites are
common in subtidal ramp sediments.
The plate displays fabrics of tempestites formed under high-energy (proximal) and low-energy (distal) condi-
tions.
Proximal
tempestites (-> 1, 3, 4) are often bioclast-dominated and coarse-grained, and composed of many
amalgamated and composite beds.
Distal
storm beds (-> 2, 5, 6) are thin-bedded and mud-dominated.
The pictures 1-3 and 5 on this plate show thin sections of a
case study
. The Late Triassic limestones formed
in the intraplatform setting of the
Kössen Basin
(Northern Calcareous Alps) contain many limestone beds inter-
preted as tempestites (Kuss 1983).
The samples are from a sequence consisting of alternating marls and limestones. Sedimentation took place in
subtidal low-energy environments of a protected platform influenced by storm events. Some of the limestone
beds exhibit erosional sedimentary structures (gutter casts and pot casts), cross bedding, lamination and ripple
marks as well as distinct accumulations of shells. The latter occur together with thin, fine-grained peloidal
grainstone layers. Field criteria and microfacies suggest high- and low-energy storm sedimentation. High-en-
ergy deposition is indicated by the deposition of complete and disarticulated shells (-> 4). Low-energy deposi-
tion by the lamination textures of fine-grained peloidal grainstones (-> 4, 6). The shell accumulations (lumachelles,
coquinas) are caused by storm-induced bottom flows.
1
Tempestite
. Brachiopod coquina formed by storms. An interpretation as
proximal tempestite
is supported by the preserva-
tion of complete shells, the current orientation of shells as well as a sharp lower erosive boundary and an irregular upper
boundary. Brachiopod shells are geopetally infilled with micrite. The lower part of the figure shows a mudstone matrix
with parallel laminae composed of very small peloids and thin compacted shells. The joint occurrence of these low-energy
laminae and the high-energy fabric of the coquina layer points to rapidly changing depositional conditions. Late Triassic
(Kössen Formation, Rhaetian): Loferbachgraben, Salzburg, Austria.
2
Distal tempestite
bed intercalated within calcimudstone (CM). Both the lower and the upper surface of the tempestite are
flat. Slight compositional differences occur from bottom to top: a - densely-packed wackestone with abundant recrystal-
lized involutinid foraminifera (white) and small peloids; b - wackestone with bivalve shells, involutinid foraminifera and
peloids; c - wackestone with sparse bivalve shells and peloids. The involutinid foraminifera indicate sedimentation as
platform-derived sediment. The low-angle cross-bedding boundary between a and b (arrows) suggests the effect of strong
currents in the upper flow regime. Same locality as -> 1.
3
Proximal tempestite.
Storm layer intercalated in lime mudstone. A cross-bedded coquina (CBC) at the base is overlain by
thin parallel laminae (GL) grading up into mudstone (M). The discontinuous lens-like geometry of the laminae suggests
deposition from unidirectional flows. The arrows point to the distinct erosional basal boundary. Note the chaotic texture
of the upper coquina consisting of disarticulated bivalves and gastropods (G). Broken shells point to deposition of mate-
rial that has been fragmented prior to the storm event. Late Triassic (Kössen Formation, Rhaetian): Gaissau near Hallein,
Austria.
4
Storm layer
intercalated within bioclastic shelf carbonates adjacent to organic mud mounds (see Pl. 143). The tempestite
layer differs from the background sediment by the composition and grain size. From bottom to top: a - bioclastic wacke-
stone (below and above the tempestite layer) with fine-grained skeletal debris consisting of ostracods, styliolinids, trilo-
bites and echinoderms, b - coarse-grained echinoderm packstone representing the lower part of the tempestite layer.
Arrows point to syntaxial rim cement around echinoderm fragments. c - bioclastic packstone with echinoderms and
tabulate corals (TC). Note the stromatactis-like fabric (S) at the upper boundary of the storm layer. Compare Pl. 94/7 and
Pl. 143. Early Devonian (Emsian): Erfoud, southern Morocco.
5 Concentration of thin-shelled bivalves and a few gastropods within
the distal tempestite
bed. Densely-packed, well-
sorted, bioclast-supported fabric. The carbonate mud is only preserved as post-storm infillings between the bioclasts.
Compare the very similar fabric formed by trilobite shells shown in Pl. 94/7. Late Triassic (Kössen Formation, Rhaetian):
Steinplatte, Tyrol, Austria.
6 Uppermost part of a thin
distal tempestite
layer. The parallel orientation of the shells indicates control by weak bottom
currents. Note similarities with -> 2. Middle Triassic (Hauptmuschelkalk): Garnberg, Künzelsau, Germany.
-> 1-3: Kuss 1983
