Stratigraphic evidence for Late Ordovician (pre-Hirnantian) glaciation, central
Nevada
Mark Tyra, Maya Elrick, Viorel Atudorei
Previous work suggests that there were two major pulses of glaciation in the Late
Ordovician—the late Mohawkian (Chatfieldian) and the well-documented Hirnantian
event. We examined a composite section of the Upper Ordovician Hanson Creek
Formation in the Monitor Range in central Nevada to evaluate the paleoclimatic history
between these two glacial intervals and identify and interpret the origins of 3rd-order (1-5
My-scale) sequences.
Five 3rd-order sequences (30-100 m thick) were identified between the Chatfieldian and
Hirnantian events. The transgressive systems tract of each sequence is composed of
basinal lime mudstone and shale and the highstand systems tracts, and sequence
boundaries are composed of progressively shallower water wackestones (Sequences 1
and 2) to packestone-grainstones (Sequences 3, 4, and 5). No evidence of sequence
boundary unconformities was observed. With cross-bedded skeletal grainstones and
debris flow conglomerates, the top of Sequence 3 possesses a sedimentological imprint
essentially identical to that observed at the Hirnantian glacial event, which overlies
Sequence 5. Preliminary conclusions from these sequence stratigraphic trends suggest
that extensive My-scale sea-level changes were occurring at least 8 My before the
Hirnantian glaciation, which implies that Gondwanan glaciers were present and
fluctuating in volume prior to the well known end-Ordovician glaciation. Samples
obtained from these five 3rd-order sequences are being processed to determine the Oisotope composition of apatitic conodonts to substantiate these preliminary glacial
interpretations.
The Copenhagen Canyon section represents a deeper water facies and possesses more
continuous deposition than the shallower Basin and Range sections. This section consists
of lower slope-to-basinal mudstone and shaley mudstone overlain by shallower, near
wave-base, bioturbated and/or shelf-edge detrital deposits at third-order sequence
boundaries. A total of five pre-Hirnantian third-order sequences are identified here, one
with almost the same sedimentological imprint as the Hirnantian glaciation itself. The
first three sequences are progressively shallower followed by a deepening in sequences
four and five. Preliminary conclusions include the possibility that extensive sea-level
changes were occurring throughout the Late Ordovician, indicating that Gondwanan
glaciations were present and long-lived. Samples obtained are being processed to
determine oxygen isotopic compositions of apatitic conodonts during this time period to
further explore this possibility.