麻豆传媒

Sedimentological research on the Scots Bay Member of the McCoy Brook
Formation at Wasson Bluff provides evidence of lacustrine facies that rest
unconformably on the North Mountain Basalt. Since the first vertebrate fossils were
found at Wasson Bluff in 1976 by Paul E. Olsen, the site has been the focus of
extensive paleontological research. The Scots Bay Member extends as a series of
micro-basin successions for several kilometres. Although attributed to a lacustrine
or playa setting, a detailed Sedimentological study is needed to constrain this
interpretation.

The initial 10m of strata above the basalt were measured in a trench on the
beach. The basal 1.9 m corresponds to the Scots Bay Member, which fills the uneven
topography on the basalt surface. The lowermost fine- to-medium-grained redbrown
sandstone is overlain in turn by red mudstone, grey-green mottled siltstone,
ostracode-rich biomicrite (5 cm and 12 cm beds) and further red mudstone. The
member is overlain by red fluvial sandstone with dinosaur bone fragments.

In a cliff separated from the trenched area by faults are three lithofacies not
observed in the trench, vertebrate-bearing purple-grey fine-grained sandstone,
green sandstone, and nodular limestone (a single bed 12 cm thick). The purple-grey
sandstone, draped over basalt clasts, contains abundant, densely packed semionotid
fish material. The nodular limestone has a disrupted fabric with discontinuous
concave-up laminae of varied colour, sediment-filled cracks, and minor continuous
red-brown laminae. This lithofacies contains abundant pale nodules of sparry calcite
that increase in proportion upwards and layers with matrix-supported ostracodes.
Disrupted fabrics are present also in the grey-green mottled siltstone, red
mudstone, and ostracode-rich biomicrite in the trenched section. Minor fish
material is present in the mottled siltstone, red mudstone, and nodular limestone
facies.

The sedimentology, sequence stratigraphy, and taphonomy of the Scots Bay
Member imply an extensive shallow lake that ponded on the basalt in the earliest
stages of basin subsidence after the eruption. Correlation of the measured sections
suggests the initial filling of an ~2.5 m depression on the basalt surface in this area.
The fish-rich sandstone marks a transgressive lag, and the ostracode-rich biomicrite
marks the most offshore and probably deepest conditions. Disrupted fabrics
indicate periodic drying up and the topmost red mudstone may represent a
regressing shoreline. The green sandstone and nodular limestone facies represent
isolated playa ponds on top of the basalt that developed during second-order
transgressive-regressive cycles. Dinosaur fragments in the Scots Bay Member and
overlying fluvial strata imply transport of bone into the lake and deposition in a
shoreline facies.

Keywords: Minas Basin, lower McCoy Brook Formation, North Mountain Basalt,
playa, lake, Semionotus, taphonomy, sedimentology
Pages: 117
Supervisors: Martin Gibling / Tim Fedak