Exhumation of the Coast Mountains Batholith during the Greenhouse to Icehouse transition in Southeast Alaska: A multidisciplinary study of the Paleogene Kootznahoo Formation
What: This study will use a multi-disciplinary approach to unravel the depositional history of the Kootznahoo Formation in Southeast Alaska with a specific focus on the exhumation history of the Coast Mountains batholith (CMB), and how high latitudes (~57°N) recorded overall global cooling from the Paleocene-Eocene thermal maximum (PETM) through the Eocene-Oligocene transition to the present icehouse state.
When: June 11 – July 5, 2009
Where: We will meet in Petersburg, Alaska and spend a few days field tripping to become acquainted with Coast Mountains geology. Most of our time will be spent working out of Kake, Alaska, a small Tlingit village on the Northwest coast of Kupreanof Island.
Who: 9 Students and Professors Cameron Davidson (Carleton College), Karl Wirth (Macalester College), and Tim White (Penn State University).
Project Description and Goals
The Paleogene is perhaps one of the more enigmatic times in Earth history marked by relatively rapid carbon (δ13C) and oxygen (δ18O) isotope excursions in deep sea sediments (Zachos et al., 2001), extensive volcanism (Courtillot & Renne, 2003), elevated levels of extraterrestrial material (e.g. 3He; Farley, 1998) and rapid uplift and exhumation of the Coast Mountains batholith in North America (Gehrels et al., in review). How these various events are recorded in the rock record and their impact on global climate has received a considerable amount of interest in recent years (e.g. Zachos et al., 2008). In Southeast Alaska (Figs. 1 & 2), the Kootznahoo Formation was deposited in a marginal marine fluvial to paludal environment throughout most of the Paleogene (Dickinson et al., 1990) and during the exhumation of the Coast Mountains batholith (Karl et al., 1999; Gehrels et al., in review). The Kootznahoo Formation is primarily composed of conglomerate, sandstone, and shale. Locally, coal seams up to one meter thick are present, shale horizons contain deciduous leaf impressions, and carbonized wood fragments and tree stumps are preserved (Lathram et al., 1965; Dickenson and Vuletich, 1990). These observations support the growing body of evidence that Earth was considerably warmer near the poles during the Paleocene and Eocene. In addition, conglomerate and sandstone from the Kootznahoo Formation contain abundant lithic fragments, feldspar, zircon, and other heavy minerals (e.g. garnet) suggesting that the source region for much of the detritus is from the adjacent igneous and metamorphic rocks of the Coast Mountains batholith complex.
For this study, we propose to use a multi-disciplinary approach to unravel the depositional history of the Kootznahoo Formation in Southeast Alaska with a specific focus on the exhumation history of the Coast Mountains batholith (CMB), and how high latitudes (~57°N) recorded overall global cooling from the Paleocene-Eocene thermal maximum (PETM) through the Eocene-Oligocene transition to the present icehouse state. The Kootznahoo Formation crops out discontinuously for over 100 kilometers from Angoon in the north to Zarembo Island in the south (Fig. 2). Our focus will be on the area easily accessed from Kake, Alaska in the protected waters of Keku Straight. If weather and time permit, we will also try to reach the outcrops north of Frederick Sound.
One of the primary goals of the project will be to better establish correlation between the various exposed sections of the Kootznahoo Formation (Fig. 2) and to specifically determine the age relationships between the different sections. Does the present day distribution of the Kootznahoo Formation reflect differential erosion of a once laterally continuous basin? Or, were these rocks deposited in smaller isolated basins? To help answer this question we will carefully measure and describe the stratigraphy of the Kootznahoo Formation from different locations, paying particular attention to sedimentary structures for paleoflow measurements, and for marker beds that can be used for correlation. In addition, selected sections will be targeted for detailed magnetostratigraphy to help with correlation and timing of deposition. Our work will build on the preliminary stratigraphic correlations made by Tim White, Peter Haeussler, and Sue Karl (manuscript in prep).
Previous work in the area (e.g. Dickenson and Pierson, 1988; White et al., in prep) and some reconnaissance work by Davidson and Wirth in 2005 show that the arkosic sandstones found throughout the Kootznahoo Formation contain abundant zircon, garnet, and other heavy minerals. These detrital minerals were probably derived from the exhumation of the adjacent CMB complex. At this point, the age of the Kootznahoo Formation is not well constrained but is reported to range in age from Paleocene through Miocene based largely on three distinct flora assemblages (Lathram et al., 1965). If we can firmly establish the age of deposition of the Kootzahoo Formation through magnetostratigraphy and radiometric ages on interbedded volcanic flows near the top of the section (Fig. 3), we have an opportunity to record the timing of surface exposure of various plutonic and metamorphic rocks from the CMB by measuring the age distribution of detrital zircons from within the Kootznahoo Formation. The timing for such a study is ideal thanks to a recent regional compilation of zircon and titanite dates for the CMB by Gerhels et al. (in review) who show distinct spatial trends in the amount (magmatic flux) and timing of pluton emplacement. This compilation will serve as a database to allow the correlation of detrital zircon populations in the Kootznahoo Formation with the ages of known plutonic events in the CMB.
We will do our best to match student interests and home institution capabilities to the various potential projects listed below.
- Detailed stratigraphy, sedimentology, and provenance (6-9 students). Most, if not all the students will participate in measuring and describing the stratigraphy and sedimentology of the Kootznahoo Formation at various locations. Sections will be chosen based on access, continuity, and appropriate lithologies that can be used for magneostratigraphy, paleontology, and detrital zircon studies described in more detail below. One or more students might choose to focus entirely on measuring sections, collecting paleocurrent measurements, and sampling for detailed descriptions and provenance studies of important units, or stratigraphic marker beds that can be used to correlate between the various sections. Provenance studies will be an important part of unraveling the unroofing history of the Coast Mountains batholith.
- Magnetostratigraphy (4-6 students). This project will focus on collecting high-resolution sampling (1-5 m) for detailed magnetostratigraphy for selected sections in the Kootznahoo Formation and interbedded volcanic rocks. The primary goal of this project is to help constrain the age of these rocks by comparing the magnetic reversal stratigraphy with the geomagnetic polarity time scale of Cande and Kent (1995). Magnetic measurements will be completed in consultation with Josh Feinberg at the Institute for Rock Magnetism at the University of Minnesota, and/or David Evans at Yale University. Students who come from institutions close to one of these labs, or can arrange their own travel, will have the opportunity to collect their magnetic data in summer or fall, 2009.
- Carbon Isotope Stratigraphy (2 students). We hope to be able to collect carbonaceous mudstone, wood fragments (Fig. 4), and carbonate samples with sufficient spacing to give us high-resolution chemostratigraphy of carbon and oxygen isotope values across the PETM and E-O boundary. These data will be used to compare the stable isotope stratigraphy of a high-latitude basin in North America (the Kootznahoo) with that recorded in marine records and other terrestrial basins, and will give us another stratigraphic tool for correlation between sections. Organic matter and carbonate samples will be run at Penn State under Tim White’s supervision.
- Paleontology/Paleoclimatology (1-2 students). Fossilized wood (Fig. 4), leaves, and coal are common in the Kootznahoo Formation and could be used to help constrain the climate conditions during deposition. The ultimate direction this project will depend on student interest and availability of home school expertise. Examples include counting preserved leaf stomata to quantify pCO2, quantify leaf shapes to help determine mean annual precipitation, or perhaps even tree-ring studies in preserved stumps.
- Detrital zircons (2-3 students). Fine to coarse grained sandstone and conglomerate are common throughout the Kootznahoo Formation and should yield abundant zircon. A cursory examination of an arkosic sandstone collected on Zarembo Island (Fig. 2) by Davidson and Wirth in 2005 reveals zircon and garnet in thin section. After detailed mapping, logging of stratigraphic sections, and making tentative correlations between sections in the field, this project will focus on collecting a few targeted samples for detrital zircon analysis. Preliminary work by George Gerhels (pers. com.) suggests that most of the sediments are derived from immediately adjacent plutons. If this preliminary finding holds up, it suggests that the Kootznahoo formation formed in small, isolated basins during the exhumation of the CMB. Macalester and Carleton have all the necessary equipment for zircon separation and characterization including cathodoluminescence imaging. Zircon dates will be collected in consultation with George Gerhels in the Laserchron lab at the University of Arizona in summer or fall, 2009. This lab funds undergraduate student travel and housing to work in the lab for NSF-funded projects, and George has agreed to help us in any way he can.
- Igneous Geochemistry (2 students). The Kootzahoo Formation is cut by bimodal dikes and is locally inter-bedded with basaltic and rhyolitic flows near the top of the section (Karl et al., 1999). The goal of this project is to characterize the whole rock geochemistry using major, trace, and perhaps radiogenic element data to help define the source region and tectonic setting for these rocks. In addition, we will attempt to extract zircon from volcanic flows within the Kootnzahoo stratigraphy to help constrain the magnetostratigraphy. Macalester College is fully equipped for measuring major and trace element data.
Southeast Alaska is a temperate rainforest. It rains a lot in a rainforest, so participants must be prepared with complete rain gear and ready for relatively cool temperatures. The average high in June and July is around 60°F and it typically doesn’t go below 40°F. Our primary mode of transportation is a skiff, a small boat with a 25 or 40 hp outboard motor, and most of our field work is at or near sea level. Therefore, you must have a good pair of Wellies, rubber boots with a good sole. These will be your primary footwear and are critical for happy and productive geologists working in SE Alaska. Therefore, you must have a good pair of Wellies, rubber boots with a good sole (fig 5).
In Kake, we will stay in an “overflow lodge” owned by the local tribal council where we will be able to sleep, cook, work on maps, and have group meetings. The lodge has two bathrooms and one shower and most of us will need to sleep on the floor so everyone will need a sleeping bag and pad. Our daily routine will include packing lunches and leaving for a day of fieldwork using skiffs. We will have three skiffs with a faculty driver for each skiff. All participants will be required to wear personal floatation devices and we will be trained in boat safety and handling while doing shoreline geology. White, Wirth, and Davidson all have experience in Southeast Alaska working with skiffs. There is near 100% exposure along the shore with a large tidal range (up to 7 meters) that allows spectacular access to the geology. We will be able to communicate with each other and Kake via cell phones and VHF radios. Each boat will have a marine GPS unit with built in tide tables. Our day will finish with dinner and a group meeting to discuss progress and scientific goals for the next day.
Coursework in one or more of the following is recommended: sedimentology, stratigraphy, petrology, paleontology, geophysics. Experience in the field in inclement weather and comfort working out of small boats with outboard motors in marine waters subject to high tidal range is desirable.
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