Geology Reference
In-Depth Information
hardgrounds with ferruginous-phosphatic crusts are
common. Bedding is disturbed by slides and slumps.
Calcarenite and breccia intercalations as well as tur-
bidites are common. The pelagic sequences are inter-
preted as being deposited at relatively shallow depths
on the top of and adjacent to fault-bounded seamounts
formed by drowned carbonate platforms (Colacicchi
and Baldanza 1986).
Similar pelagic deep-water facies and association
with the shedding of shallow-water material from car-
bonate platforms into adjacent open-marine basins oc-
cur in the Cretaceous of the Middle East and Mexico
(Wilson 1975; Enos 1977).
Fig. 15.37.
Chalk.
The SEM photograph, a detail of Pl. 7/3
exhibiting a soft chalk, shows the accumulation of intact
coccoliths and microcrystalline calcite particles, most repre-
senting disintegrated coccolith plates with varying degrees
of calcite overgrowth. Interparticle microporosity is slightly
reduced. Porosity reduction is largely due to dense packing
of coccoliths and microcrystalline calcite particles resulting
from consolidation (mechanical compaction) rather than from
a large amount of cement crystals formed during weak com-
paction. Consolidation results from an increase in grain-pack-
ing density and concurrent expulsion of pore fluids. Consoli-
dation is the main process by which pure calcite oozes, with
an initial porosity of about 80%, are transformed into soft
chalks with porosities between 40% and 50%. Late Creta-
ceous: Deep Sea Drilling Program Site 463, core 44, western
equatorial Pacific.
C
halk: An exceptional pelagic sediment
Chalk is a friable or hard pelagic lime mudstone
containing planktonic and benthic foraminifera (see
Sect. 10.2.2.1), calcispheres (see Sect. 10.2.1.9), bi-
valve fragments (chiefly
Inoceramus,
Pl. 87/8), echi-
noid plates, and locally bryozoans and other fossils.
The fine fraction consists of up to 75% planktonic biota
(coccolithophorids, calciodinoflagellates; Pl. 7/3-5),
and calcite crystals representing disintegrated nanno-
fossils and cement crystals (Fig. 15.37). Burrows and
trace fossils suggest substrate stability and the avail-
ability of sufficient oxygen near the sediment-water
interface. Extensive hardgrounds, glauconite and phos-
phatic mineralization are common. The lower parts are
rich in clay and marls, and cherts. Resedimentation (in-
dicated by common millimeter- to centimeter-sized in-
traclasts; Pl. 16/5) and intercalations of allochthonous
carbonates are abundant in many chalk sections, e.g.
in the North Sea chalk (Schatzinger et al. 1985). Most
of the Cretaceous chalk of Europe, North America and
the Middle East was deposited during global sea-level
highstands in open shelf environments and in deep-sea
settings.
Microfacies data of chalks
are important because of
the poor visibility of sedimentological and biogenic
structures in large parts of chalks. Maliva and Dickson
(1992) studying the Cretaceous/Tertiary chalks of the
Eldfisk Field in the Norwegian North Sea differenti-
ated four microfacies defined by grain composition and
fabric, abundance and distribution of authigenic min-
erals, and evidence of the mechanisms of porosity re-
duction:
•
•
Extraclast-Bonetocardiella-foraminifera wacke-
stone,
characterized by planktonic foraminifera and
calcispheres, and transported chalk clasts indicat-
ing debris flow deposits.
•
Extraclast-foraminifera packstone/wackestone
char-
acterized by foraminifera, extraclasts, sponge spi-
cules, radiolarians, thoracosphaerid and other calci-
spheres, benthic foraminifera, ostracods, echino-
derms, and detrital quartz grains. Many of these con-
stituents also occur as minor components in the other
microfacies. The extraclast-foraminifera packstone/
wackestone microfacies constitutes most of the tur-
bidites within the chalks.
Pelagic carbonates deposited on the ocean floor
Pelagic sediments with an inferred oceanic basement
have been described from various parts of the world,
e.g. the Cretaceous Troodos Massif of Cyprus and the
Franciscan Complex of California (see review in
Jenkyns 1986), but the only reliable guide to the rec-
ognition of true oceanic sediments is an association with
ophiolites. Most of the unequivocalocean-floor facies
are Jurassic or Cretaceous in age. Bernoulli and Jenkyns
(1974) distinguished five facies types:
•
Foraminifera mudstone
characterized by volumetri-
cally predominant planktonic foraminifera.
•
Radiolarian-foraminifera mudstone,
differing from
the foraminifera mudstone microfacies only by com-
mon molds of radiolarians that are empty or filled
with calcite, pyrite or silica.
Ophicalcites. Brecciated surface of the oceanic base-
ment (serpentinite, gabbro) heavily brecciated. Fis-
