Environmental Engineering Reference
In-Depth Information
3.2.1 Ichnology in the Delineation of Cycles
Hoffman (2000)
recognized a hierarchy of cycles within the lowermost 46 m of
the Fort Terrett Formation in the vicinity of Kerrville, Texas, composed of 15
meter-scaleHFCs, 4 high-frequency sequences (HFSs) with thicknesses of about
7-18 m, and 1 or 2 composite sequences. The BurrowedMember of the Fort Ter-
rett Formation is included in the 15 cycles of
Hoffman (2000)
.
Kerans (2005)
placed the Fort Terrett Formation within a single Albian composite sequence.
Eight HFCs (
Fig. 9
) were delineated within the Burrowed Member of the
Fort Terrett Formation at the Real County study area; they are of two types:
upward-deepening and upward-shallowing cycles (
Fig. 9
A). Four upward-
deepening HFCs (A-D,
Fig. 9
A) occur in the lowermost part of the Burrowed
Member and are overlain by four upward-shallowing HFCs (E-H,
Fig. 9
A). The
eight HFCs compose part of the transgressive and the highstand systems tract of
one provisional HFS (HFS A,
Fig. 9
A) and part of the transgressive systems
tracts of a second provisional HFS (HFS B,
Fig. 9
A). Combined, the eight HFCs
indicate an overall landward shift of a paleoshoreline within the context of a
transgressive systems tract included in a composite sequence (
Fig. 9
A).
The lower boundary of HFS A, which is exposed southwest of the road cut
shown in
Figure 9
A, corresponds to the contact between the Glen Rose Forma-
tion and the Basal Nodular Member of the Fort Terrett Formation (
Rose, 1972
),
a regional composite sequence boundary (
Kerans, 2005
). The maximum flood-
ing surface of HFS A is at the upper bounding surface of HFC D (
Fig. 9
A). HFC
G is mainly composed of micrite recrystallized to microspar and dolomite. The
upper surface of HFC G forms the upper surface of HFS A. The top of HFC G in
the West Frio section of
Rose (1972
: 181-182) is riddled with rhizoliths, which
indicates that the upper boundary of HFS A is associated with a relative fall in
sea level and subaerial exposure. A prominence of
Thalassinoides
, mudstone to
grainstone depositional textures, peloids, miliolids, and bivalve fragments are
found to characterize much of the Burrowed Member of the Fort Terrett Forma-
tion and are highly suggestive of a low-energy, restricted shallow-marine, shelf
interior (cf.
Rose, 1972
). A monospecific ichnotaxon and paleoenvironment
express a
Thalassinoides
-dominated
Cruziana
Ichnofacies for much of HFS
A. An open shallow-marine depositional facies is characteristic of HFS B
(
Fig. 9
A), as suggested by a substantial presence of green algae, and caprinid
and toucasid rudists (
Rose, 1972
). Only a very few dense accumulations of
Tha-
lassinoides
were observed in HFS B (
Fig. 9
A). A general upward, landward shift
in depositional facies fromHFS A to HFS B indicates a backstepping of the HFS
B open shallow-marine shelf over the HFS A-restricted shallow-marine shelf
interior. A more regional delineation of ichnofabrics and paleoenvironments
should be useful for long-distance correlation of cycles within the Burrowed
Member of the Fort Terrett Formation.
The stratiform
Thalassinoides
-dominated ichnofabric is mostly restricted to
the micrite-rich parts of the HFCs of HFS A, where it is characterized by an
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