EOS 330
Landon Mutch
Oct. 9, 2014
Muir Creek Field Trip Report
The beachfront outcrop at Muir Creek on the west-coast of Vancouver Island, see Figure 1, contains
many well preserved fossil horizons. The outcrop is part of the Sooke Formation, which belongs to the
Carmanah Group , and based on analysis of its molluscan, vertebrate, and invertebrate fauna is dated
from late Oligocene to early Miocene [1].
Figure 1.
Muir Creek beach outcrop near Sooke,
BC, modified from GoogleMaps.
The cross section of the outcrop along the beach strikes mainly east-west with horizontal bedding layers
also striking east-west, see Figure 2. Though it is difficult to get a good measurement of dip, the bedding
is lying roughly flat and may be dipping very gently to the south [2]. The lithology is predominantly finemedium grained sandstone that often grades into, or is abruptly replaced with, coarse to pebbly,
sandstone-lithic conglomerate layers with minor interspersed layers of siltstone often containing black
organic material, see Figures 3-4.
Figure 2.
Rough diagram of the beach outcrop looking
northward, with specific locations marked with
circled numbers.
Figures 2 – 5 are taken from my field notebook and detail some of the more prominent structural
features of the outcrop such as fossil horizons, bedding, and faulting.
Figure 3.
Location 1 – showing a normal fault
offset by approximately 1m and
striking 156/30. Bedding consists of
layers of sandstone, conglomerate,
siltstone, coalified organic matter,
with interspersed fossil layers
generally appearing in well-defined
horizons.
The interspersed lithologies of sandstone, conglomerate, and organic-rich siltstone indicate a
depositional environment frequently changing back and forth from higher energy rocky coastline to
medium energy sandy shore to low energy backshore lagoon environments. The fossils are largely intact
with minimal signs of erosion and breakage and often suddenly appear on the bottoms of sandstone
beds which indicates they were likely deposited offshore during lower energy cycles.
Figure 4.
Location 2 – well defined bedding
layers laterally continued from
location 1 showing large (up to
4cm) gastropod and bivalve fossils.
Bedding is cut through by large
(~4cm wide) gravel-filled burrows
which indicate they belong to the
trypanite ichnofacies [2].
Often bedding shows fining up
sequences, cross-bedding dipping
shallowly to the east, and pinching
out of finer laminations to the east
and west.
Location 3 – higher oxidation of
iron in beds indicated by distinct
reddish color.
The changes in depositional environments through this outcrop are likely a result of period rising and
falling of sea level which would have caused an associated movement of the shoreline. The sediment
layers at this particular outcrop will reflect those shifts in depositional environment. For example, a
regression event may change and sedimentary sequence from rocky coastline to sandy beach or lagoon
environment and promote deposition of a well preserved fossil horizon; while a transgression event may
shift an offshore medium energy sandy cross-bedded environment to a higher energy rocky shoreline
where trypanite burrows penetrate and sediments and are eventually filled in with course clastic
material.
Figure 1.
Location 4 – two opposing normal faults, with the
left showing cm scale offset and the right showing
m scale offset, indicating regional extension.
Location 5 – some barnacle fossils.
To scale drawings of gastropod fossils in vertical
and horizontal cross section as well as casts and
molds of shells.
To scale drawings of bivalves in cross section, with
some lying concave upwards and others concave
downwards indicating low to medium energy
depositional environments.
The most abundant and obvious fossil beds contain predominantly bivalve and gastropod fossils which
have been preserved by recrystallization of shells from aragonite to calcite as well as by molds and casts
of shells [2]. The gastropod and bivalve fossil shells can easily be distinguished by their morphologies:
gastropods have spiral shaped shells, while bivalves have shallow cup-shaped shells (originally two
joined halves, which have subsequently been separated), see Figure 5.