Geology Reference
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of some significance only in reef carbonates and fore-
reef breccias. These fissures correspond to cavities be-
tween large reef-building organisms or reef blocks. The
cavities are largely filled at an early stage by sediment
derived from the erosion of the reef itself (Land and
Goreau 1970).
Synsedimentary fissures
develop by downslope slid-
ing and slumping of early-cemented slope deposits on
uncemented interbeds as a result of earthquake shak-
ing (Martire 1996). Shallow-marine lagoonal carbon-
ates can be affected by desiccation and shrinkage lead-
ing to the formation of dikes (Fischer 1964).
Tectonic fissures
are often related to block faulting
and rifting. Regional tectonics associated with active
block faulting and extensional movements causes the
formation of faults and gravity sliding of large sedi-
mentary blocks along unstable slopes due to oversteep-
ening (Füchtbauer and Richter 1983; Winterer and Sarti
Plate 24 SedimentFilled Fissures: Neptunian Dikes and Terrestrial Infills
Vertical, horizontal or oblique millimeter- to meter-sized fissures filled with carbonate or other sediment or with
carbonate cements can be formed in submarine and subaerial environments. Neptunian dikes are 'discordant
dikes of limestones within limestones' filled with submarine sediments. Fissures are common in platform car-
bonates, particularly in drowned platforms (-> 4) and at platform margins, but also occur in slope and pelagic
limestones (-> 1, 2, 6). Submarine formation of fissures is triggered by synsedimentary tectonics and rifting or
differential compaction. Subaerial fissure formation occurs during karstification (-> 5) and vadose dissolution.
Internal fillings in neptunian dikes consist of mud, pelagic and benthic fossils and lithoclasts eroded from fissure
walls. Microbial crusts can occur.
Common cements are early-marine radiaxial-fibrous calcite and later granular calcite covering the walls of
fissures. Common fillings of paleokarst fissures are erosional carbonate lithoclasts and relicts of overlying de-
posits, silt, and pedogenic sediment, carbonate cements are of meteoric-phreatic and burial origin.
Analysis of neptunian dikes requires one to study the geological framework, compare the microfacies of
fissure sediments and host rocks, evaluate fossils (taphonomy, biotopes, age) and examine the diagenetic crite-
ria. Common microfacies types of submarine fillings are red and gray mudstones and wackestones/packstones
containing foraminifera, mollusk and echinoderm coquinas and lithoclasts. The timing of fissure fillings can be
determined by microfossils, particularly foraminifera. Differences in texture reflect open or closed fissure sys-
tems. Alternations of cement tapestries and sediment infill reflect repeated rupture of fissures. Neptunian dikes
indicate breaks in sedimentation and changes in sea level and subsidence. Low sedimentation leads to condensed
sequences with condensed faunas, covering a substantial time period.
1
Neptunian dike
(outlined in white) within red pelagic limestones with 'filaments' (pelagic bivalves). The host rock is a
filament wacke/packstone, the filling is a fossiliferous mudstone. Note the distinct difference between the microfacies of
the host rock and that of the infilling sediment. This difference and the irregular wavy boundary of the fissure (white line)
indicate a modification of the fissure system by submarine dissolution and a certain time break. Deep-water limestone.
Late Triassic (Carnian): Hydra Island, Greece.
2
Internal filling
of a horizontal neptunian dike. The alternations of grain-rich and grain-free laminae and aligned shells
point to the existence of bottom currents within the dikes. Jurassic (Rosso Ammonitico Veronese): Altopiano di Asiago,
Italy.
3
Repeated rupture and fissure formation.
Two generations of fissures (outlined with white ink) in a red pelagic wacke-
stone. An older system is cut by younger fissures filled with radiolarian packstone. Late Triassic (Norian): Epidavros,
Greece.
4
Submarine fissures
in drowned platform carbonates. The Late Triassic host rock (peloid grainstone, PG) is cut by repeat-
edly ruptured fissures exhibiting several phases of Liassic fillings: 1: Red echinoderm wackestone, 2: Red mudstone,
3: Calcite vein cutting the red mudstone, 4: Gray crinoid packstone. The arrow points to the youngest fissure. Triassic/
Jurassic: Kirchbruch, Adnet, Austria.
5
Fissure fill
. Terrestrial fissure within Jurassic shallow-marine bindstones. The karst fissure has been filled with late Cre-
taceous angular to subrounded limestone clasts. Note the plane boundary of the fissure and the cracks filled with burial
calcite cement. Late Jurassic (Ernstbrunn Limestone, Tithonian): Ernstbrunn, north of Vienna, Austria. The Ernstbrunn
limestone is part of a Late Jurassic-Early Cretaceous carbonate platform which was affected by intensive karstification
prior to a Late Cretaceous transgression (Hofmann et al. 1999).
6
Pelagic wackestone
(PW) with filaments, radiolarians and
Globochaete
, cut by synsedimentary subvertical Q-fissures,
infilled by fine-grained sediment and juvenile mollusks (M). Note bivalve (
Halobia
) coquina, C) at the bottom and thin
calcite tapestries at the walls of the fissures. Late Triassic (Hallstatt limestone, Norian): Bad Ischl, Upper Austria.
-> 1, 3: Dürkoop et al. 1986, 2: Martire 1996
