Geology Reference
In-Depth Information
• Facies change surfaces characterized by marked
microfacies contrasts, differences in grain sizes and sig-
nificant differences in diagenetic patterns.
• Marked landward shift of distal facies over proxi-
mal facies.
• Hardgrounds (Sect. 5.2.4.1). Surfaces coated by
mm-thick crusts of iron oxide.
• Condensation surfaces (Sect. 5.2.4.2).
• Borings (Sect. 9.3). Surface with extensive borings
may indicate interruption in sedimentation or non-depo-
sition and characterize a flooding surface.
• Phreatic cements.
• Breccias. Sedimentary breccias can mark underly-
ing sequence boundaries (Biddle 1984; Garcia-Mondjar
1990; Sect. 5.3.3.5).
• Hardground intraclasts indicate breaks in sedimen-
tation, often connected with maximum flooding sur-
faces.
• Oncoids are common at the base of transgressive
sequences of low-energy environments and can mark
maximum flooding surfaces (Wright 1983).
• Palynomorphs. Because palynofacies parameters are
linked more or less directly to the distance of the coast
line and/or water depths, a rapid marine flooding will
cause a significant signal in the composition of the pal-
ynofacies assemblage (Rameil et al. 2000).
• Organic matter and bioturbation. Intensive burrow-
ing suggesting low sedimentation rates, and concen-
tration of organic matter may indicate the relatively
deepest environment and the maximum flooding sur-
face.
cessions. Siliciclastic intercalations and terrestrial de-
posits are common as well as mixed siliciclastic-car-
bonate deposits.
Grains are predominantly bioclasts. Normal marine
biota but skeletal grains reflecting adaption to brack-
ish-water or restricted conditions are common (e.g. os-
tracods; miliolid foraminifera). Ooids scarce. Extra-
clasts (Sect. 4.2.8.2) may indicate an increased drain-
age of river systems caused by sea-level lowstands.
Black pebbles are good markers of sea-level lowstand
phases connected with subaerial exposure (Box 4.22).
Transgressive Systems Tracts
Platform interior well flushed and normal marine.
Reefs and sand shoals narrow, with wide passages.
Patch reefs occur far landward. TSTs develop deepen-
ing-upward and increasingly open-marine trends. Typi-
cally thin-bedded carbonates. Non-skeletal grains, e.g.
ooids and peloids, tend to be more common in TSTs
and HSTs, because their growth is triggered by tidal
currents and extensive winnowing by waves on the
flooded platform top. Intraclasts common. Skeletal
grains diverse, comprising benthic foraminifera, mol-
lusks, echinoderms, and algae. Lime mud rare.
Highstand Systems Tracts
Reefs and sand shoals at the platform margin com-
mon and wide and more continuous. Shoaling-upward
facies successions. Restriction increases in platform
interior environments. HSTs are characterized by an
Box 16.2.
Preliminary guide to identifying systems tracts
in a single section or borehole. After Schlager (2002).
The identification must start with the knowledge of the
position of the section with respect to the long-term plat-
form edge. This knowledge may be derived from re-
gional geological information, clues from the use of
Standard Facies Models or the evaluation of fossil as-
semblages.
Section location seaward of the platform edge:
• LST: All shoalwater intercalations in slope deposits.
• TST: Slope deposits with platform debris low, decreas-
ing upward in the section.
• HST: Slope deposits with platform debris high, increas-
ing upward in the section. Ooids, peloids, platform-de-
rived mud and specific platform interior skeletal grains
abundant.
Section location landward of the platform edge:
• LST: Represented by exposure surfaces and terrestrial
deposits.
• TST: Deepening upward and increasingly open-marine
trend.
• HST: Shallowing upward and increasingly restricted
trend.
Differentiating LST, TST and HST of platforms and
ramps
Generally carbonate systems tracts are identified
from geometric relations derived from shore-to-basin
cross sections recorded in seismic profiles, large out-
crops or well-correlated boreholes. However, there is
an increasing demand to recognize sequences and sys-
tems tracts in outcrop sections or single boreholes as
well. Some microfacies criteria are listed below.
Schlager (2002) suggested a preliminary guide for iden-
tifying systems tracts shown in Box 16.2. This approach
is speculative, but may become more reliable by using
selected microfacies criteria as shown by the case stud-
ies in Sect.16.1.2.3.
Lowstand Systems Tracts
Shoalwater belts are much narrower than transgres-
sive and highstand tracts. They are well flushed, rich
in reefs, hardgrounds and rocky shores.
LSTs are characterized by shallowing and deepen-
ing-upward successions forming meter-sized facies suc-
