Environmental Engineering Reference
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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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