Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

quadrangle are intimately associated with some of the northwest- to north-northwesttrending
faults that may have Laramide heritage. However, the strong association of BR
zones with the two structures that bound the western margin of the Upper Arkansas
graben and South Arkansas graben (Shavano fault zone and Willow Creek fault,
respectively) suggests that the BR zones may be related to Neogene faulting associated
with the rift. If the BR zone shattering was produced during repeated Neogene faulting
that juxtaposed Proterozoic rocks on the west with the Tertiary Dry Union gravels on the
east then some mixing of the Tertiary gravel clasts with the BR zones might be expected.
However, there is a complete lack of a rounded pebble component and a lack of the
lithologic diversity that typifies the Dry Union conglomerate clasts in the BR zones,
especially those segments that are directly adjacent to the Dry Union Formation.
It is possible that the BR zones represent remnants of the earliest rift fault breaks
that are preserved in the stranded walls of the fault zones and that the Dry Union gravels
have been juxtaposed during younger faulting events that have stepped inward (with
respect to South Arkansas graben) and away from the earlier faults. Another possibility is
that the spatial arrangement of BR zones with the west edge of the grabens may be
fortuitous and could be explained by marginal rift bounding faults using and reactivating
older planes of weakness related to Laramide faults. Another less likely interpretation is
that the BR zones are somehow related to the emplacement of the detached Paleozoic
blocks/sheets (Td2ls) that were incorporated in the lower part of the South Arkansas
graben Dry Union Formation (Td2) section. However, the lack of shattering in the
Paleozoic outliers and the linear distribution of the BR zones and the indication they are
localized along steeply dipping fault zones does not support this interpretation.
BR zones are relatively subtle features and can easily be overlooked. Some of the
more important aspects of the zones that could be used to help constrain their origin are
the distribution pattern and timing. So far they have only been found cutting Proterozoic
rocks. It would be very important to determine if BR zones have a broad regional
distribution pattern in the Proterozoic terrane or if they are spatially related to the rift.
They also might be localized in the structurally complex area of intersecting faults,
horsts, and grabens at the south end of the Upper Arkansas graben. The preliminary
observations presented here suggest they are at least locally intimately associated with
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