2014 Pacific Section SEPM Fall Field Trip

PACSEPM Fall Feild TripCONTINENTAL EXTENSION OLD AND NEW AT THE EDGE OF THE MOJAVE

November 8-9, 2014

FIELD TRIP OVERVIEW

This year’s Pacific Section SEPM Fall Field trip will look at an excellently exposed section of the Meso-Neoproterozoic Pahrump Group (pre-Cryogenian Crystal Spring, Horse Thief Springs and Beck Spring Formation, Cryogenian Kingston Peak Formation and Ediacaran Noonday and Ibex Formations) in the Silurian Hills, as well as newly-described Miocene rock avalanche megabreccias in the northern Salt Spring Hills, with the goal of understanding the challenges of how to fit the deposits into a broader basin analysis context. Detailed correlations with regions outside the Mojave are challenged by complex faulting, magmatic overprinting, and a poor understanding of original basin geometry.

The Silurian Hills and Salt Spring Hills are found at the northern border of the Mojave Desert, at the eastern terminus of the Garlock Fault and south of Death Valley. During the Meso-NeoProterozoic, this region shifted from one of intracratonic platform to the leading edge of a new passive continental margin (Laurentia, formed from rifting of the older Rodinian supercontinent) .

North and northeast of the Silurian and Salt Spring Hills, , Neogene extension has exposed large swaths of time-correlative Proterozoic strata in such well-studied locales as the Panamint Range, Saddle Peak Hills, Kingston Range, and Alexander Hills. To the east lies the Shadow Valley, the location of spectacular Neogene extensional basins with thick sedimentary deposits, including enormous megabreccia sheets representing far- traveled rock avalanches. South and southeast, significant exposures of Mesoproterozoic (and rare Paleoproterozoic) basement complexes of the Mojave Province are found in low relief hills. Recent crustal thinning in the area enabled extensive basaltic volcanism and young cinder cones and flows found throughout the area. Directly south and west of the area, late Permian through Cretaceous granitoids formed during the arc magmatism which obliterated the older rock record, although greenschist-grade windows of metasediments are preserved.

In contrast to the extensive normal-faulting within the Basin and Range province, the Mojave Desert is dominated by strike-slip faults and lateral translations which make direct correlations difficult. Similarly, understanding the geologic setting during major Miocene unroofing and extension is challenged by lateral translation, magmatism and uplift, and lack of appreciation for long run-out rock avalanches. . The Salt Spring Hills are the westernmost expression of the megabreccia deposits well-described from the nearby Shadow Valley area and Valjean Hills.

PS-SEPM Fall Field Trip 2014 Announcement (PDF)

Register Now for the PS SEPM Fall Field Trip

Devonian Carbonate Platform of Eastern Nevada: Facies, Surfaces, Cycles, Sequences, Reefs, and the Cataclysmic Alamo Impact Breccia.

Pacific Section SEPM Fall Field Trip, October 4-­‐6 2013

Field Trip Leaders: John E. Warme (Professor Emeritus, Colorado School of Mines) and Mara E. Brady (Assistant Professor, Fresno State, mebrady@csufresno.edu)

REGISTRATION & LODGING RESERVATION DEADLINE: SEPTEMBER 20

FIELD TRIP LIMIT: 50
Participants PLEASE NOTE that student registration is limited to 8 students per college/university

Mail in the Registration Form Below

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REGISTRATION FORM

Fall 2013 Field Trip Announcement!

FIELD TRIP OVERVIEW

PS-SEPM Fall Field Trip: October 4-6, 2013

DEVONIAN CARBONATE PLATFORM OF EASTERN NEVADA: FACIES, SURFACES, CYCLES, SEQUENCES, REEFS, AND THE CATACLYSMIC ALAMO IMPACT BRECCIA.

Trip Leaders: John E. Warme (Evergreen, Colorado) and Mara E. Brady (Fresno State, mebrady@csufresno.edu)

Stratigraphic column

Stratigraphic column showing ~250 m of carbonate platform mostly cyclic limestones and dolostones from the upper part of the newly named Fox Mountain Formation, the informal yellow slope-forming member and shallow carbonate platform facies of the lower Guilmette Formation, the newly named Alamo Breccia Member of the middle Guilmette, and the lower beds of post-Breccia limestones and sandstones of the informal upper Guilmette (from Jared R. Morrow, after Sandberg, Morrow and Warme, 1997).

Devonian limestone and dolostone formations are superbly exposed in numerous mountain ranges of southeastern Nevada. The Devonian is as thick as 1500 m there and reveals continuous exposures of a classic, long-lived, shallow-water carbonate platform. Our two-day field excursion will be to Devonian outcrops easily reached from the settlement of Alamo, Nevada, ~100 mi (~160 km) north of Las Vegas. Emphasis will be on carbonate-platform lithostratigraphy, but includes overviews of the conodont biochronology that is crucial for regional and global correlations. These formations are mainly composed of hundreds of partial to complete shallowing-upward Milankovitch-scale cycles and are grouped into sequences bounded by regionally significant surfaces. Dolomitization in the Lower Devonian Sevy Dolostone and Middle Devonian Simonson Dolostone appears to be linked to exposure surfaces and related underlying karst intervals. The less-altered Upper Devonian Guilmette Formation exhibits characteristic shallowing-upward limestone-to-dolostone cycles that contain typical carbonate-platform fossil- and ichnofossil-assemblages, displays stacked biostromes and bioherms of flourishing stromatoporoid sponges and sparse corals, and is punctuated by channeled quartzose sandstones. The Guilmette Formation also contains a completely exposed ~50-m-thick buildup that is constructed mainly of stromatoporoids, with an exposed and karstified crest. This buildup exemplifies such Devonian structures known from surface and hydrocarbon-bearing subsurface locations worldwide.

Of special interest is the stratigraphically anomalous Alamo Breccia that represents the Middle Member of the Guilmette. This spectacular cataclysmic megabreccia, produced by a bolide strike ~382 million years ago, is as thick as 100 m where we will study it. The Breccia may be the best exposed proven impact debris field on Earth. It contains widespread intervals created by the seismic shock, ejecta curtain, tsunami surge, runoff and probable continental margin collapse generated by a major marine impact.

Travel & Logistics: Participants should plan to arrive Friday night for a Saturday 8am start of the two-day field excursion. Registration (details to be posted shortly) will include a complete printed field guide.

On Saturday, the group will visit Hancock Summit, where participants can observe and discuss (1) carbonate facies, cycles, and surfaces and (2) the Alamo Breccia and its bolide impact signatures.

On Sunday, we will visit Silver Canyon, where participants will investigate (1) the upper interval of the Alamo Breccia, which contains large clasts of platform carbonates and clasts of impact lapillistone; and (2) exceptional exposure of a carbonate buildup (Reso’s Reef), lithologies that interfinger with the buildup, and the karstified crest at the top of the buildup.