Paleoecology And Paleoclimate Of The Paleogene Chickaloon Formation, Matanuska Valley, Alaska

Matanuska_1_2012

What: The 2012 Matanuska Valley project will investigate the paleoecology and paleoclimatology of an important late Paleocene-early Eocene sedimentary sequence in south-central Alaska’s Matanuska Valley. We will focus our studies on sedimentary facies analysis, sedimentary petrology, paleoclimatic reconstructions, studies of fossil wood, fossil insect-bearing amber deposits, and studies of fossil leaf herbivory for the Chickaloon Fm. The summer program will consist entirely of fieldwork, with the expectation that some laboratory analyses will be carried out through the academic year 2012-2013.

When: July 1-28

Where: Our field sites are scattered along the margins of the Matanuska River Valley about 75 miles northeast of Anchorage, AK. The towns of Sutton, AK and Chickaloon, AK are nearby our study sites (61.71N, 148.89W). Participants will fly to and from Anchorage International Airport.

Who: Six students. Professors Christopher Williams (Franklin & Marshall College) and David Sunderlin (Lafayette College) will lead the trip.

Project goals and description

Project goals are to (1) obtain a basin-wide detailed stratigraphic and sedimentological dataset on the fluvial depositional system of the fossil-bearing Chickaloon Formation, (2) reconstruct the paleoclimatic history and paleoecological succession of forested paleoenvironments in that stratigraphic context, and (3) provide field instruction and primary research experience to a motivated group of undergraduate students.

Tentative Student Projects

  • Spatial lithofacies variation in the Chickaloon Fm. (1-2 students). Thus far, detailed stratigraphic architecture of the unit is only known from the exposure of upper Chickaloon material at Wishbone Hill. Our project would expand our studies of sedimentology and stratigraphy to document the lateral variation of both lithofacies and fossil plant communities throughout the basin. Bedding plane outcrop exists throughout the Matanuska Valley for minimally time-averaged, snapshot views of the variability of sediment deposition. As described below, these sediments preserve diverse fossil leaf mats. Thus, potential student projects focused on detailed stratigraphic studies at numerous outcrops can be integrated with quantitative fossil plant studies. Students will measure vertical sections at intervals and develop stratigraphic models among the measured sections with the goal of producing a history of the fluvial stratigraphic architecture to provide a comprehensive view of the unit.
  • Paleobotany (1-2 students). The upper Chickaloon Formation is known to be rich in well-preserved fossil leaves. Leaf physiognomic analyses of fossil leaves from several stratigraphic layers in the upper Chickaloon Fm. have proven useful for estimating paleoclimate (mean annual temperature and precipitation; Sunderlin et al., 2011). Thus, we envision a student project that will expand on this work, perhaps collecting fossil leaves across variable lithofacies or at different stratigraphic intervals to refine estimates of early Eocene paleoclimate from this deposit. A complementary study would be a taxonomic analysis of the fossil flora including work with fossil cones, seeds, and fruits that are commonly encountered in the deposit.
  • Ancient Plant-Animal Interactions (1 student). Some of the fossil leaves described above
exhibit insect damage including hole-feeding,
margin feeding, surface feeding, and
skeletonization damage types (Sunderlin et al.,
2011). Given the utility that insect
damaged fossil plants have in reconstructing
paleoenvironmental change
and the paucity of such data from high
paleolatitudes near the Paleocene/Eocene boundary, bed-by-bed data from the Chickaloon Fm are desirable both for within-formation paleoecological study and broader continental-scale analysis in context with other data. We envision a student will conduct this analysis on whole leaf and leaf fragment collections made from bench quarries in the same beds that are selectively sampled for taxonomic diversity and leaf physiognomic study.
  • Amber with floral and faunal inclusions (1 student). An outcome of our 2008 Keck project was the discovery of an insect-bearing amber locality from the measured stratigraphic section at Wishbone Hill. Given the record of fossil leaf damage in the Chickaloon flora, arthropod inclusions have the potential to be extraordinarily informative because the association between an insect record within the same strata as leaf herbivory data is, to our knowledge, unique worldwide. Dispersed amber is abundant and easily collected from mire facies in the Chickaloon Fm. We envision a project in which a student collects plant fossils with amber inclusions as well as unassociated amber from throughout the vertical and horizontal sedimentary sequence. Analytical techniques (e.g., FTIR, pyrolysis-GS-MS), if available at the students home institution would be used to characterize the organic chemistry of the amber to define biomarker compounds from amber in identifiable and unassociated amber. A study of the paleontology of arthropod inclusions in the amber would also be possible through an existing collaboration with the American Museum of Natural History.
  • Fossil tree growth (1 student). Insight into tree growth and the variability in growing conditions can be gained by detailed analyses of growth ring trends in fossilized wood This is of interest because some Chickaloon fossil woods exhibit “false-rings” which indicate interrupted wood formation during the growing season. Such rings may have multiple causes (e.g., floods, frost, defoliation) indicative of sub-optimal growing conditions. The tree growth data, ring analyses, and lithofacies data could be used in concert with the paleoclimate studies proposed above to determine the response of trees to climate and disturbance in these paleoecosystems. Our work in 2008 demonstrates the abundant well-preserved fossil wood exists in the Chickaloon to conduct these studies (Williams et al., 2010).
  • Geochemistry and chemostratigraphy (1-2 students). In 2008 we collected a stratigraphic sequence of bulk organic matter from measured stratigraphic sections at the Wishbone Hill site in order to evaluate the variation in stable carbon isotope abundance in the upper Chickaloon Formation (Neff et al., 2011). We identified a moderate negative carbon isotope excursion (-3.5‰ δ13C PDB) within a 1 m section of transitional floodplain to lacustrine sediment. This excursion is of similar magnitude to that found in other Paleocene/Eocene successions. A student project could be to refine the carbon isotope chemostratigraphy in the Wishbone Hill section. This would involve high-resolution sampling of organic matter at the stratigraphic interval containing the carbon isotope excursion. This study could be broadened to collect samples of pedogenic carbonate for isotopic analysis to gain insight in the paleohydrology of the Chickaloon Fm.
  • Geochronology and Sedimentary Petrology (1 student). Volcanic ash partings can be observed in coals in the upper Chickaloon Formation and are common elsewhere in the unit. These have been altered to bentonite and kaolinite clay and they form thin persistent layers that are easily recognized at our localities. Triplehorn et al. (1984) recovered zircons from the Chickaloon Formation that should be suitable for modern, high-precision U/Pb isotopic dating. Moreover, the Chickaloon Formation is relatively rich in channel deposits of lithic sandstones, which are amenable to detrital zircon analysis. We envision one student project that might gather stratigraphically-tied provenance and airfall tuff geochronological data within our previously logged stratigraphic sections in order to provide good age control. This student project would be encouraged if the student’s home institution advisor had expertise in such geochronological methods. A study of the sedimentary petrology and geochemistry of these beds would also be helpful to the project goals.

Working Conditions

Matanuska_2_2012_smallFieldwork for this project will be concentrated at old coal mine exposures and along roadcut and river bank outcrops in the Matanuska Valley (about 1.5 hours drive northeast of Anchorage). Additional study of related strata may be made by helicopter (pending funding) or on foot to remote and isolated backcountry exposures along tributaries to the Matanuska River. Participants should be comfortable working and hiking on rugged terrain. July weather in south-central Alaska is highly variable. High temperatures can range from mid 40s to high 80s (F). Rainy and misty days are very common and so are active mosquito populations. Both may last for many days at a time. Black and grizzly bears as well as moose are occasionally present in the field areas.

Accommodations will be mostly in tents at primitive campgrounds (pit toilets and no running water) or at the field site (no toilets or running water). Electricity and refrigeration are non-existent at the field and campsites. We will plan a few breaks throughout the trip to do laundry, ship samples, and reprovision. These breaks may include the occasional night in a rustic cabin setting with showers. Food preparation will take place in the field and consist of simple backpacking-type meals. We will generally self-cater our meals in town. Cell phone service is spotty in the Matanuska Valley but both AT&T and Verizon work (non-3G) in the smaller towns along the Glenn Highway.

Required Gear

Beyond the obvious clothes/toiletries/prescription medications, you’ll want:

  • Sleeping bag (you should have good one rated at 20 degrees or below)
  • Sleeping pad (we can lend these out if need be)
  • Personal tent (two person size; we have some loaners if you don’t have one to use)
  • Daypack
  • Warm clothes (winter coats are overkill, just a fleece is underkill. We like a softshell jacket for middle of the road weather.
  • Gloves, warm hat
  • Sunglasses
  • Boots that come up to ankles (to keep rocks out of shoes)
  • Rain gear (waterproof, not water resistant; jacket and pants) (LLBean & EMS have good, 
lightweight stuff for decent prices)
  • Pants that you donʼt mind sitting in the dirt with. Jeans, Carhartts, or some other sturdy ones.
  • Big duffel bag or backpack in which to jam all this stuff.

Course Preparation

Matanuska_4_2012_smallStudents should have completed the junior year and have had at least one course in sedimentology/stratigraphy or paleobiology/paleontology. Previous field experience in a field camp or field research setting is highly desirable. Courses in historical geology, biology, botany, organic chemistry, mineralogy, petrology, or geomorphology will be helpful.

References

  • Neff, J.L., Hagadorn, J.W., Sunderlin, D., Williams C.J. 2011. Sedimentology, facies architechture and chemostratigraphy of a continental high-latitude Paleocene-Eocene succession-The Chickaloon Formation, Alaska. Sedimentary Geology, 240: 14-29. (doi: 10.1016/j.sedgeo.2011.07.002) [pdf]
  • Sunderlin, D., Loope, G., Parker*, N.E., Williams, C.J. 2011. Paleoclimate and paleoecology implications of a Paleocene-Eocene fossil leaf assemblage, Chickaloon Formation, Alaska. Palaios, 26(6): 335-345. [pdf]
  • Triplehorn, D.M., Turner, D.L., and Naeser, C.W., 1984. Radiometric age of the Chickaloon Formation of south-central Alaska: Location of the Paleocene-Eocene boundary. Geological Society of America Bulletin, 95: 740-742.
  • Williams, C.J., Trostle, K.D., Sunderlin, D. 2010. Fossil Wood in Coal-forming Environments of the Late Paleocene-Early Eocene Chickaloon Formation. Palaeogeography, Palaeoclimatology, Palaeoecology 295: 363-375. [pdf] (doi:10.1016/j.palaeo.2010.02.027).

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