“The Origin of the Strjugsskard Andesite,
Skagi Peninsula, North-Central Iceland”

by

Charlene Adzima

Iceland is located in the North Atlantic Ocean between Greenland and the British Isles. Being situated on both the Mid-Atlantic Ridge and a mantle plume, Iceland has been the site of a voluminous and complex volcanic system over the past 16 million years. Excess magmatism driven by the mantle plume has buoyed up the Icelandic crust so that it is the only sub-aerial exposure of a mid-ocean ridge. Interaction between the plume and ridge systems has produced rocks that span the compositional spectrum from basalts to rhyolites. The origin of the intermediate rocks, andesites, is ambiguous in Iceland. They have either been formed as a result of fractional crystallization or mixing between a mafic end member and a silicic end member.

An anomalous plagioclase megacryst-rich andesite was identified in the course of the 2003 Iceland Keck project in the Skagi Peninsula region of north-central Iceland. Based on initial sampling, this unit was preliminarily interpreted to reflect magma mixing between plagioclase ultraphyric basalt (PUB) and dacite magmas. I conducted a detailed field and laboratory study of this unit.

The Strjugsskard andesite (SA) lava thicknesses range from 31 to 84 m. The overlying pyroclastic units related to the SA range from ~2 to 25 m thick. SA was divided into lithologic types according to plagioclase megacryst abundance. Normal SA contains 5-20% megacrysts, sparsely porphyritic SA contains about 1-5% megacrysts, and very sparsely porphyritic SA contains less than 1% megacrysts. The megacrysts of all samples show a wide range of degree of sieve texturing and the edges are usually rough and ragged, indicating disequilibrium. Electron microprobe analyses have shown that the megacrysts in both the PUB and SA have a fairly uniform composition of An82-An86, which is a composition that is in equilibrium with basaltic magma, not andesite or dacite.

In general, the lower abundance of megacrysts has a higher silica content (rhyolite to dacite) and the higher abundance of megacrysts has a lower silica content (andesite). Variation diagrams of major, minor and trace elements show that the Strjugsskard andesite lies on straight line mixing trends between PUB and a silicic end member. Trace element plots helped to refine the silicic end member, showing it to be a narrow range of dacite to low-silica rhyolite. Based on these results, SA was formed by mixtures of 10-60% PUB and 90-40% dacite/rhyolite. I suggest that PUB magma both erupted on its own and intruded into a zoned silicic magma chamber, mixing with the denser dacite and some low silica rhyolite before it was erupted as Strjugsskard andesite.

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“Paleoenvironments and Stratigraphy of the Middle
Ordovician Strata of Central Tennessee”

by

Suzanne Boyenton

The Middle Ordovician strata of central Tennessee are well known for their nearly complete units and abundant marine fossils. Using samples collected from these strata, I further interpret the paleoenvironments and biostratigraphy of central Tennessee using previous depositional models as a guide. I have similar interpretations to Holland & Patzkowsky (1998), differing only in the interpretation of the upper member of the Murfreesboro Limestone. It was originally interpreted as representing a peritidal facies; the interpretation now is a deep subtidal facies that is similar to that shown by the Pierce Limestone which overlies the Murfreesboro Limestone. Paleontological material is a dominant feature in these strata including a possible algal form present in the upper Lebanon Limestone and the bottom of the Lower Carters member. Oobiosparite concentrated in the upper Lebanon Limestone is hypothesized as having had the highest energy depositional environment in this sequence and it possibly represents an ooid shoal within the shallow subtidal environment. The sequence of the paleoenvironments from deep subtidal to shallow subtidal is characteristic of a marine transgression.

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“A New Dendroclimatic Tree Ring Network from Lake Clark
National Park and Preserve, Alaska”

by

William W. Driscoll

Many dendroclimatic studies have been conducted in Alaska to aid in understanding recent climate changes, identify past and current warming trends, and determine how climate change may influence regional ecosystems. Four new white spruce (Picea glauca) ring~width chronologies from a 30 kilometer north-south transect from as early as AD 1500 to 2003 from the Lake Clark National Park and Preserve in Southern Alaska were developed. Three chronologies showed positive responses to warming after 1950 and strong correlations with May-July average monthly temperatures from King Salmon, Alaska. The fourth chronology was less sensitive to growing season temperatures after 1950 and did not correlate significantly with average monthly temperatures for any part of the year. The chronology contained a bimodal distribution of both strongly positive and strongly negative responses. All series in the chronology correlated strongly prior to 1950 and diverged soon thereafter, suggesting that highly localized conditions affected growth response. Regional variability often attributed to differing climate influences may be due in part to different local growing conditions. Population~wide analyses that ignore the possibility of divergent responses may not present an accurate assessment of recent warming in the context of past conditions.

A principal component analysis was conducted using three chronologies and the positive component of the fourth chronology to reconstruct temperatures on a regional scale. April~July temperatures of one and two years prior to growth were used; all eight predictors passed at the .05% confidence level and the model explained 30.82% of the variance. The model showed a clear warming trend during the 20th century, which accelerated after the mid-1970s until present.

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“A revised Holocene stratigraphy of Long Island, Bahamas,
and its relation to global sea level change”

by

Drew Feucht

The stratigraphy of the Bahamas Islands has been used as a sea level proxy because of the area’s tectonic stability and high sedimentation rates. Previous studies show that the end of the Pleistocene on the islands is marked by a calcrete paleosol and then overlain by transgressive grainstones of the Holocene. Long Island, the geology of which has not received much attention, has a stratigraphy that can be correlated to the surrounding islands by morphological and sedimentological similarities. There are unique stratigraphic features on Long Island, however, like possible Coccothrinax argentata palm tree trunk molds within the North Point Member. There are also conflicting and controversial radiometric dates from fossils for the Pleistocene/Holocene boundary.

The paleosol on the Bahamas Islands is believed by most geologists to have marked the end of the Pleistocene (last glacial interval) and the beginning of the Holocene (the present interglacial time), with all units above it forming during a global sea level transgression. However, after 14C dating Cerion shells (a terrestrial gastropod) within the unit above the paleosol and obtaining a date of 36 ka ± 1 ka, this chronostratigraphic conclusion must be reconsidered. The paleosol, as well as the unit directly above it, are here considered to have formed during the Pleistocene. All units above this unit were deposited during the transgressive seas of the Holocene, with landward-moving dunes overtaking an ancient Coccothrinax argentata palm tree forest.

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“Geophysical Study of Northern Cyclone Canyon Graben in Canyonlands National Park, Utah: Sediment Depth and Implications for a Cross-Sectional Model”

by

Joshua Michaels

The arcuate grabens of the Needles fault zone in Canyonlands National Park, Utah, were the location of the Keck Canyonlands 2004 project, which employed exploration geophysical techniques to determine the Quaternary sediment depth in Cyclone Canyon graben. Four seismic refraction profiles from northern Cyclone Canyon are analyzed and discussed here. The extensional structure of the Needles fault zone is anomalous given that it occurs in the stable Colorado Plateau tectonic province. However, the Needles fault zone can be explained as a brittle sedimentary rock block extending westward on the ductile Pennsylvanian Paradox Formation (predominately evaporites) into the void created by the Quaternary downcutting of the Colorado. This extension creates regularly spaced grabens on the eastern side of the Colorado River. Numerous previous studies of these grabens have assumed a Quaternary sediment depth of less than 25 meters for any graben. Grosfils et al. (2003) published an exploration geophysical study of Devil’s Lane graben concluding that the maximum sediment depth was about 80-90 meters. Based on results presented here, Cyclone Canyon has a maximum sediment depth of 75 meters. Cyclone Canyon is about twice as wide as Devil’s Lane. Assuming that the initial width of the grabens was similar, a standard textbook model of a graben does not account for new data for Cyclone Canyon graben (seismic refraction, gravimetry, observations from the field and digital maps). A step model, introduced by Moore and Schultz (1999), explains these observations by internal faulting within an initial graben block, forming “steps” and producing shallow sediment depth relative to graben width, and a reactive (or possibly active) salt diapir that would produce a larger negative gravity anomaly than expected if only sediment depth affected gravity.

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“A New Family and Genus of Bivalve from the Triassic of
Southern Israel and its Paleoenvironmental and
Paleoecological Context”

by

Allison C.S. Mione

The bivalve Paceconcha ramanensis (Brotzen 1956) represents a new family and genus of bivalve found in the Middle Triassic strata of southern Israel. The new family is Paceconchidae and is found within the Superfamily Megalodontoidea. Paceconcha replaces the genus Myalina, which was used by Brotzen (1956) for this specimen. P. ramanensis (Brotzen 1956) is only found in the Upper Member of the Gevanim Formation within Makhtesh Ramon. The organisms were epifaunal to semi-infaunal filter feeders. They rested on their broad, ventral surface. In order for them to accomplish this, they needed a flat plane on which to sit. The animals lived in the shallow-marine conditions that existed in the region during the Middle Triassic. The area was positioned over the equator with the depositional environment found on the northern coast of Pangea. The region experienced a tropical climate with warm, fairly clean waters which allowed the animals to live successfully in the aragonitic seas of the time. The Triassic deposits were buried under millions of years worth of strata until the Middle Cretaceous. The unit where Paceconcha ramanensis (Brotzen 1956) is found has been traditionally known as a reef. My research has shown that this not likely. There is no evidence within the deposit that the animals were ever connected to form a wave-resistant barrier. Their life mode also makes it impossible for them to have grown and lived connected to each other. The organisms show no preference within the unit to lend evidence that they constituted a reef. The deposit is still very unusual and will require further study to determine how the animals carne to rest in such an odd shape and isolated location.

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“Geophysical Studies of Southern Cyclone Canyon Graben,
Canyonlands National Park, Utah”

by

Amanda Trenton

Cyclone Canyon graben is one of many small, geologically young (60-65 ka) grabens of the Needles District in Canyonlands National Park, Utah. The grabens developed by faulting of jointed brittle Permian sedimentary rocks extending over the ductile Pennsylvanian Paradox Formation (evaporites) toward the void created by down cutting of the Colorado River. Seismic refraction profiling reveals that the sediment depth to the bedrock floor of the graben ranges from 11 to 40 meters for southern Cyclone Canyon graben, the results of Joshua Michaels and his survey of northern Cyclone Canyon graben increase the depth to the sediment bedrock interface to 70 meters. Based upon this two-layer seismic profiling model, resulting from travel time curves, the seismic velocities are ~500 m/s for the Quaternary alluvium fill in the graben and 2,000 –2,500 m/s for the bedrock of the graben. The sediment thickness, when combined with scarp height, yields a displacement of ³110 meters on the master fault of Cyclone Canyon graben. Based on refraction seismology profiles, southern Cyclone Canyon graben has an abrupt, deep southern termination, shaped by the influence of deep-seated joints of the Chesler Canyon lineament. Supporting the conclusions of Grosfils et al. (2003), seismic results from Cyclone Canyon graben indicate that previously published estimates of less than twenty-five meters of Quaternary graben filling sediments in Canyonlands grabens are incorrect. Approximation of depth to the sediment-bedrock interface allows a preliminary estimate of 192 meters of total extension accommodated in the area near Cyclone Canyon graben of Canyonlands National Park.

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“Using Tree-Ring and Glacial Records from Southern Alaska to Investigate a Possible Temperature Structure to the First Millennium AD”

by

Nicolás E. Young

Paleoclimatic work done in the Gulf of Alaska region has focused mainly on the last 1000 years. Due to an extensive glacial and tree-ring record there is a relatively well-defined climatic chronology for this interval. In contrast, there is a limited record that exists from the First Mi11ennium AD (FMA).

In an effort to calendar date a floating tree-ring series that has been radiocarbon dated to the FMA, 25 tree-ring series from Columbia Bay, Alaska were added to an existing Columbia Bay tree-ring record. The hope was that these new series would cross a gap between the calendar-dated Columbia Bay chronology and the FMA ring-width series. In this study, the gap was not crossed, however, significant gains were made with respect to investigating a possible temperature structure in the FMA for southern Alaska. This was accomplished by analyzing existing radiocarbon-based floating tree-ring series for their climatic significance. The Columbia Glacier chronology was not only extended to AD 581, but also strengthened due to the addition of these 25 series.

In addition to adding to the Columbia Bay tree-ring series, seventy-nine ring width series from Beare, Sheridan, and Tebenkof glaciers in the Gulf of Alaska were analyzed to infer a temperature history for this interval. This chronology suggests a cooling trend during the FMA that is comparable to the cooling during the Little Ice Age. This finding is consistent with glacial studies from the region that indicates significant cooling during the FMA.

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“Reinterpretation of the Zohar Formation of Makhtesh Qatan
(Callovian, Middle Jurassic): Parting the Geology of the Holy Land”

by

Kevin R. Wolfe

The Zohar Formation (Callovian, Middle Jurassic) contains within it two units which, based on field observations and hand samples, have been determined to be a set of previously undescribed limestone rocky shorelines. These units record two highstand events in which early cemented limestone units were exposed as a rocky shoreline during the Jurassic. While exposed, tidal channels were excised into these units following joints formed at a shallow depth. Dolomitization, which has occurred to both of the channelized units, occurred after their respective shoreline stages. The tidal channels in this exposure had been previously considered “mega mudcracks”, formed as part of a large sabkha. This interpretation has been set aside with the discovery of the ichnofossils Thalassinoides suevicus and Planolites isp, which are made in shallow marine environments, on the sole of the upper channelized unit. Overall the Zohar Formation represents a eustatic sea level drop from the Sherif Formation below it.

Wooster Petrology in Central Park, April 2005
(Left to Right): Andy Horst (’07), Charlene Adzima (’05), Monica Umstead (’06), Cordelia Dennison-Budak (’07), Anton Heitger (’06), Jessica Hark (’07), Brian Mumaw (’07), Will Driscoll (’05),
Jeff Bowen (‘06), and Emily Cunningham (’07)