37-21 Laramide Structural Influence on the Preservation of Dinosaur Bones, Morrison Formation, Central Colorado

Session: Geoscience Research Poster Showcase by 2YC and 4YCU Undergraduate Students

Poster Booth No.: 144

Presenting Author:

Jemma Petrie

Authors:

Petrie, Jemma¹, Simonsen, Adam², Sonheim, Ryan Jacob³, King, Michael Ryan⁴, Petrie, Elizabeth S.⁵

(1) Geology & Geological Engineering, Colorado School of Mines, Golden, CO, USA, (2) Geology, Western Colorado University, Gunnison, CO, USA, (3) Geology, Western Colorado University, Gunnison, CO, USA, (4) Geology, Western Colorado University, Gunnison, CO, USA, (5) Geology, Western Colorado University, Gunnison, CO, USA,

Abstract:

Fractured bones of a Haplocanthosaurus were recovered from the Gordon-Bramson-Brothers Quarry in the lower part of the Jurassic Morrison Formation south of Snowmass, Colorado. These fractures were previously postulated to be the result of Pleistocene ice wedging within shallowly exhumed strata. This study tests that hypothesis by comparing fracture orientations measured in quarry-adjacent outcrops with those observed in bone fragments.

Structure-from-Motion photogrammetry was used to collect fracture orientation and spacing data and reorient previously quarried fossils. Fracture planes cutting the bones were mapped using the compass tool in CloudCompare. Fracture data obtained from structural transects at outcrops in the Jurassic Morrison/Curtis Formations and Entrada Sandstone show regularly spaced fractures (Cv = 0.4-0.6) that dominantly strike north with dips ≥50°. Fractures mapped within the bones have parallel to subparallel orientations (±11°) and regular fracture spacing (Cv = 0.4-0.6), consistent with the fracture spacing observed in sandstone and limestone outcrops.

A chevron bone could be reoriented using its position on the published quarry map by applying a 22° clockwise rotation. After restoration, fractures within the chevron strike east (mean strike 85° ± 11°) and dip steeply south (60-85°).

Thin-sections from outcrop and bone fragments show that veins formed in the subsurface. Outcrop samples contain both extensional and shear veins that cut and locally offset ooids and quartz grains, as well as evidence of pressure solution and cataclasis. In the bone fragments, veins crosscut mineralized pore spaces, indicating that at least one phase of mineralization predated fracturing. Calcite veins in both outcrop and bone samples contain mechanical twins, indicating deformation under elevated confining pressures and temperatures in the subsurface. Evidence of Laramide structural influence in the study area includes steeply dipping beds (>60°) and nearby faults that cut the Paleozoic and Mesozoic units. The regular fracture spacing observed in both outcrop and fossil bone suggests a strata-bound fracture system, potentially formed during Laramide uplift and tilting prior to surface exposure. However, nearby faults may have locally influenced fracture orientations.

This study demonstrates a method for restoring fossil orientation for geologic analysis. When adequate quarry documentation exists, fossil fragments can be reconstructed to extract fracture data. Petrographic evidence suggests that the bones were mineralized and fractured in the subsurface and potentially exploited by later Pleistocene freeze-thaw processes.

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.