AEG field trip: Back to the Tonian!

Notice of Field Trip
At the Lehigh Cement Mill & Quarry,
Union Bridge, MD
Announcement from the BWH Section of the
Association of Environmental & Engineering Geologists
Saturday, October 27, 2012
10 AM – 3 PM
A Trip Back to the Tonian: 960 Ma Volcanism and Microbialites
in the Westminster Terrane of Maryland
Liz Graybill and Bob Ganis
ABSTRACT: The Sams Creek/Wakefield Complex in the Westminster Terrain of Maryland includes highcalcium,
low-magnesium limestone exploited in the vicinity of Union Bridge for cement production.
Stratigraphic, geochemical, petrographic and radiometric dating analyses of new exposures revealed an
elegant geologic history.
The stratigraphic studies showed that the interlayered carbonates, metabasalts, phyllites, and rare
sandstone channels were contemporaneously deposited. Continuous deposition of carbonate rocks
containing biogenic microbialite layers which also contain non-carbonate ash falls, lava flows, and
pyroclastic debris. Both basaltic and andesitic rocks erupted.
New zircon dates reveal a 950-970 Ma age for the sequence. These were acquired from three
units: basaltic greenstone representing the floor of the carbonate buildup, andesitic meta-ash (phyllite)
layers within the carbonate rocks, and a quartz sandstone channel within the carbonate sequence. The
metabasalt preserved 1036-1269 Ma zoned Grenville xenocrysts. The meta-ash hosted two zircon
populations: 1016-1321 Ma zoned plutonic sourced xenocrysts, and unzoned 950-970 Ma eruptive ages.
The sandstone channel preserved prismatic (944-1553 Ma) and rounded (969-1537 Ma) zircons. The
zircon populations came from both Grenville aged rocks, with zoned zircons, and from contemporaneous
volcanics, which contained unzoned zircons. No zircons younger than 944 Ma were found.
We propose that these rocks formed when Rodinia was breaking up following Grenville orogenesis,
with the development of a rift feature at ~960 Ma. The trace element composition of greenstones resembles
within-plate through plate-boundary-enriched, mid-ocean ridge basalt (EMORB), possibly tracking the
opening of a rift basin. In addition, the chemical composition of the Sams Creek meta-volcanics is similar to
other known rift volcanics. Prior hypotheses that the Sams Creek volcanics were either Catoctin related (at
~570 Ma) or were formed as Cambro-Ordovician oceanic islands with atolls (Wakefield carbonate) are not
supported by the age dates and stratigraphic relationships.
Google Map of Union Bridge & Field Trip:
Page 1 of 2
Page 2 of 2
Sample Collecting is ENCOURAGED, so bring rock hammer, sample bags, markers,
Cost: RSVP required because we need to print a guidebooks and purchase food and
beverages. $45/person (checks or cash accepted at start of field trip, or at the October
18 meeting.) Email Kris your RSVP & lunch choice.
Provided: Handout/guidebook, box lunch with sodas & water, van to visit the stops,
rock samples!
Must Bring/Wear: Mandatory hard-toed boots or shoes. Some hard hat and safety
glasses available, but bring your own so you can be styling.
Attendee Limit: 48
Box Lunch Choices:
􀀀 Ham (applewood smoked) and provolone cheese garnished with lettuce, tomato, and mayo
􀀀 Turkey breast, topped with lettuce, tomato, alfalfa sprouts, and mayo
􀀀 Roast beef (medium rare, shaved), topped with mayo, lettuce, and tomato (default if none checked!!!)
􀀀 Vegetarian – Layers of provolone cheese separated by real avocado spread, alfalfa sprouts, sliced
cucumber, lettuce, tomato, and mayo
All are an 8” homemade French bread sub topped with shredded lettuce, fresh tomatoes, and mayo. Also
includes chips, cookie, pickle.
(Turkey is default if no choice made from list of above).
To reserve, please email Kris McCandless no later than Noon, Wednesday, October 24, 2012 at and please select one of the sandwich choices above.
(Turkey is default if no choice made from list of above).
Town of Union Bridge is in Carroll County, Maryland, 21791
Meet at the Main Office of the Lehigh Cement Company, paved parking is provided.
Plant Entrance: Lat
Main Office: Lat
Chair: Ashley Hogan, GeoConcepts Engineering, Inc.,
(703) 726-8030,
Vice Chair: Steve Stokowski, RJ Lee Group, Inc.,
(508) 259-3536,
Treasurer: Drew Thomas, ECS Ltd, (703) 471-8400,
Secretary: Kris McCandless, Total Environmental Concepts,
(703) 727-4374,


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