RMAG ON THE ROCKS

» CONTINUED FROM PAGE 28 to the Virtual field trip held by RMAG on April 7th , 2022. On the virtual field trip, Brown Hawkins toured attendees around the quarry from the vantage point of a drone. Drone footage provided a unique perspective by providing time-slices of changes in the 3-dimensional geometry of quarry walls and one of the thrust faults revealed by progressive excavation.

In-person field trip attendees had previously read and certified Health and Safety documentation and everyone arrived with required personal protective equipment (PPE – hardhat, steel-toed boots and, eye protection). Reflective, high-visibility vests were distributed to all. Then, Erik Estrada (plant manager) spoke at length about the particulars of the cement manufacturing workflow. Almost all cement manufactured today is Portland cement. The AGI Glossary of Geology defines Portland Cement as “A cement made by grinding a mixture of limestone and shale (or equivalent raw materials), heating the mixture to incipient fusion in a rotary kiln and fine-grinding the resulting clinker.” (p. 509). Some of my notes from Mr. Estrada are summarized here.

The Lyons CEMEX plant:

• Is one of the smallest cement manufacturing facilities in the country. It has one of the smallest cement kilns in the US.

• Has the lowest CO2 emissions of any CEMEX facility

• Has established itself as an environmentally-friendly concrete manufacturer.

• Has a kiln that is 10’ by 250’ which uses 5 million BTUs/hr.

• Is permitted to manufacture 600,000 tons of concrete per year.

• Supplies ~25% of Colorado’s cement consumption needs.

• Does not currently capture CO2, because the plant is too small.

• Is the leading cement plant in the US for reforestation.

• Employs 120 people.

• Welcomes visitors.

After our introduction to plant operational history, we carpooled into the quarry where we stopped to look at an excellent exposure of the B-Chalk/BMarl. The upper part of the exposure was stained red because of the increased content of pyrite in the overlying A Chalk. Curiously, on the fresh surface, the chalk intervals were not easily differentiated from the marl intervals. Apparently, segregation is more easily made when the rocks have weathered. Detailed examination wasn’t possible, as this 1st outcrop was on the other side of standing water. We next drove to a location at the N end of the quarry that allowed us to get a close view of the rocks. The structural characteristics exposed in the quarry are complex. In different locations, the field trip group observed compressional, extensional and strike-slip structures. The quarry is located on the E-dipping (22°) western limb of the Dowe Pass Syncline - immediately to the W of the Rabbit Mountain anticline. These large-scale fold structures both have generally ~N-S axial trends.

In the Denver-Julesburg Basin, structural adjustment is dominated by normal faulting, with grabens galore. Stylolites in the basin are horizontal, reflecting that sigma 1 is vertical. In the quarry, however, we noted that many stylolites were vertical - reflecting compression by a horizontal sigma 1. One popular characteristic of the Niobrara here is a wide array of fracture fabrics. Many of the fractures displayed open apertures; many were mineralized, many were decorated with slicks and many showed multiple senses of shear. One wide fracture that I brought home was clotted with large calcite crystals that revealed bright white fluorescence at the planar matrix contact transitioning to mild orange fluorescence in the fracture interior.

Fossils were noteworthy and included ichnofauna from the Codell, large, bowl-shaped inoceramids from the Fort Hays and peculiar aquamarine colored “fish scales”.

All-in-all, this was a great day!

WEBLINKS

• https://www.cemexusa.com/products-and-services/cement/ product-specifications

• https://www.cemexusa.com/sustainability

• https://www.cemex.com/

• https://bouldercounty.gov/property-and-land/ land-use/cemex-dowe-flats/

4: Group photo trip participants by the drone being flown by Brown Hawkins (behind, and to the left of, the blue hard hat at upper left). Photo

5: Peculiar light blue “fish scale?”

6: The complex structure in evidence at the quarry is hinted in each of the exposed crosssections on display here. In the foreground, the thicker limestone beds of the Fort Hays show a subtle anticlinal warp to the right of the person in red. In the middle distance, the antiformal trace displays greater amplitude. In the distance, the exposed wall displays a thrust fault and a subtle rollover anticline. View to the ESE.