Estuaries - Trace Fossils as Indicators of Sedimentary Environments - page 450

here, are transgressive in nature. They receive sediment from both fluvial and
marine sources, commonly occupy the seaward portion of a drowned valley,
contain facies influenced by tide, wave, and fluvial processes, and are consid-
ered to extend from the landward limit of tidal facies at their heads to the sea-
ward limit of coastal facies at their mouths ”. We are in agreement with this
definition but modify the definition to say “from the landward limit of persis-
tently tidally modulated facies to the seaward limit of coastal facies”. This
allows the practitioner to use readily identifiable evidence for the presence
of tidally influenced sedimentation that may include one or more of the follow-
ing observations: (1) the presence of brackish-water bioturbation in the land-
ward part of the estuary, as the presence of a typical brackish-water
assemblage requires larval recruitment from the marine realm and thereby
depends upon landward transport of larvae by tidal currents; (2) the occurrence
of intertidal-flat deposits, which have a characteristic ichnological signature;
(3) the presence of bioturbated inclined heterolithic stratification (IHS), which
is consistent with landward larval recruitment and may provide evidence for
salinity-associated flocculation processes; and/or (4) the preservation of tidal
sedimentary structures. The latter observation is the most commonly used cri-
terion for recognizing paleo-estuary deposits but is likely to be the most difficult
FIGURE 1 Location of modern estuaries referred to in this chapter. Each satellite image shows the
estuary to the approximate tidal limit. The yellow stars show the locations of the reported coordi-
nates. Images courtesy of Google Earth, 2011. (A) Chignecto Bay, Atlantic Canada (45 43 0 5.66 00 N,
64 32 0 22.31 00 W), a large, tide-dominated estuary. (B) Kouchibouguac-River estuary, Atlantic
Canada (46 50 0 42.44 00 N, 64 55 0 30.75 00 W), a wave-dominated estuary. (C) Ogeechee River estuary
(31 51 0 41.88 00 N, 81 5 0 0.62 00 W), a mixed-energy estuary. (D) Willapa Bay, Washington, USA
(46 38 0 57.00 00 N, 124 0 0 10.70 00 W), a mixed-energy estuary.