Leigh van Drecht

            The Early Jurassic McCoy Brook Formation at Wasson Bluff on the northern side of the Minas subbasin has been a site of dinosaur bone discoveries for over thirty years.
Excavations at the Princeton Quarry in 1998-2006 recovered at least three articulated
prosauropods within a confined bone bed. This bone bed includes the oldest dinosaur
bones in Canada and richest prosauropod site in North America. The dinosaur material has
been well documented but the detailed sedimentology of the bone-bearing bed is of
interest for analyzing the paleoenvironment and taphonomy of the prosauropods.

A five-meter section that includes the bone-bed and overlying units was described
at a centimeter scale over the course of 10 days of field work during the 2013 excavation. Representative samples of the strata were collected for grain-size and petrologic analysis.

Laser diffraction grain-size analysis, shows that poorly sorted, fine- to medium
grained sandstone predominates. Scatter plots with combinations of skewness, mean and
sorting allowed separation of facies and support the identification of two facies
associations. In the lower part of the section, interbedded thin, red, micaceous mudstone and orange-brown sandstone show cross-beds and ripple lamination. These strata are attributed to fluvial deposition. In the upper part of the section, large cross-sets of greybrown sandstone with light and dark laminae predominate, and the sandstones are coarser and better sorted than those below. These strata are attributed to eolian deposition. Outsized, moderately rounded grains in the fluvial sandstones suggest eolian additions.

Sandstones are immature and were classified as feldspathic litharenite and lithic
arkose. During diagenesis they were minimally compacted with a clay matrix and
cementation by gypsum to form small nodules. Grains are coated by hematite and clay
giving the sandstones a characteristic red-bed appearance.

Prosauropod bones were found in two beds of trough cross-bedded sandstone in the lower part of the section and were in some cases crushed by compaction of sandstones as a result of minimal permineralization of the bones as well as being offset by small
centimeter-scale faults. Isolated basalt boulders eroded from an 8 m paleocliff face nearby
were deposited in bone-bearing beds but also occur in units that do not contain bones.

Detailed sedimentological analysis provides new information that is useful for interpreting the depositional environment of the bone bed. The results indicate that the dinosaurs were preserved in a fluvial system with episodic flow and periodic desiccation in a dune field filling a micro-basin. Dinosaurs may have been overwhelmed by flooding in a narrow basin and/or a moderate flow may have concentrated the bloated carcasses.

Keywords: Prosauropod, McCoy Brook Formation, Princeton Quarry, taphonomy, microbasin, eolian-fluvial interaction.
Pages: 79
Supervisor: Martin Gibling, Tim Fedak