In the deep interior of the Earth, dynamic processes related to convection of Earth?s mantle can uplift or subside Earth?s surface by hundreds of meters. These deflections affect sedimentation and erosion patterns, evolution of drainage systems, and water depth in marine systems on geologic timescales. Changes in the Earth?s surface topography in turn influence atmospheric circulation and biological habitats. Disentangling these diverse drivers depends upon a thorough understanding of each one. The investigators will identify the influence of dynamic processes by combining surface geological observations of erosion and sedimentation patterns in the Apennine Mountains in Italy with numerical modeling of the deeper Earth to develop a detailed understanding of how dynamic processes are preserved in the rock record. The broader impacts of this research include: full participation of women in STEM; improved STEM education and educator development; development of a diverse, competitive STEM workforce; and collaboration with science education students and in-service school teachers.

The idea that sedimentary basins that develop above retreating versus detaching slabs will have distinguishable tectonic signatures means that quantifying these differences allows for interpretation of plate boundary settings in ancient orogens. In most previous studies, slab detachment is well documented by geophysical observations that do not provide information about the timing, duration, or rate of these events. Slab rollback studies are often focused entirely on the geologic record with little to no quantitative information about the geodynamic parameters that influenced the evolution of the region. This project will focus specifically on characterizing the thermal, erosional, and sedimentary response to the transition from slab rollback to slab detachment in the northern and central Apennine Mountains in Italy. These critical geologic observations of uplift and subsidence will be used to calibrate geodynamic models that quantitatively characterize the interaction between crustal tectonics and mantle dynamics during slab rollback and detachment. The results of this project will improve our ability to detect such episodes in the geologic record and result in a better overall understanding of the evolution of long-lived convergent margins. Additionally, this study will provide better constraints on the timing and duration of basin transitions in the geologic record. The investigators will involve Masters of Arts in Teaching graduate students and in-service science teachers from around Iowa in curriculum development at the college and elementary or secondary levels. In addition, this project will provide career development opportunities for the PI through new collaborations with several international universities.

A Field Emission Scanning Electron Microscope (FE-SEM) with added diverse imaging and detection capabilities will be acquired by the University of Iowa (UI). This FE-SEM system will 1) improve the quality and range of data that will be collected and analyzed, 2) increase the speed of data collection and thereby enable more efficient and effective use of research and educational funds, 3) facilitate quantitative evaluation of robust datasets, 4) provide additional opportunities for training STEM students for the 21st Century on a robust, user-friendly analytical tool that can be easily learned with comparatively low training time, 5) provide key support for undergraduate and graduate student research and teaching across a wide spectrum of disciplines, 6) be used regularly by faculty in the Earth & Environmental Sciences Department, as well as researchers in the Chemistry and Anthropology Departments and staff in the Iowa Geological Survey, 7) deliver a new level of micro and nano imaging capability for users from the College of Engineering and Department of Physics and Astronomy, and 8) greatly enhance electron microscopy competencies at UI thereby opening new avenues of research to a wider user base within the university itself and surrounding region. Placement of the instrument into the new Iowa Materials Analysis, Testing, and Fabrication (MATFab) facility will provide critical staff and infrastructure support for the instrument, researchers at UI, easy access for researchers at UI and from neighboring institutions (Iowa State University, Cornell College) who identified as major users, as well as others who would be occasional users (University of Northern Iowa, Kansas State University, Purdue University). We envisage that this instrument will become a regional facility, attracting users from elsewhere in the Midwest, given its unique capabilities that are not available elsewhere in the region.

Addition of a Field Emission Scanning Electron Microscope (FE-SEM) with automated mineralogy capabilities will expand the scope of Earth Science research and education and transform the materials characterization capabilities of existing facilities available at the University of Iowa. Detailed chemical and textural sample characterization are increasingly critical components of many in-situ microanalytical applications in the Earth and material sciences. Correct interpretation ultimately rests on understanding the primary context of the material or grains of interest. Establishing this context involves documenting the distribution and spatial relationships of minerals or phases within a sample. Secondary electron (SE) and backscattered electron (BSE) imaging will be combined with automated energy-dispersive X-ray spectroscopy (EDS) analysis for rapid mineral identification and quantitative characterization of mineral abundance, elemental distributions, and textural properties across a broad range of materials. Cathodoluminescence (CL) imaging capabilities will greatly enhance current research efforts in trace element zoning characterization for petrologic and geochronologic studies. Simultaneous automated collection of all signals will provide cutting edge correlation of mineral distribution and textures and major, minor and trace element distributions. A field emission source will enable maximum signal generation and source longevity for mapping applications while also enabling nanometer scale imaging resolution for characterization of synthetic and natural nanomaterials. The instrument will be primarily used for applications related to geochronology and petrochronology, structural and metamorphic fabric analysis, igneous petrology, and characterization of sedimentary strata, but other applications will include economic geology, archeology and anthropology, and characterization of environmental surface films, synthesized catalytic materials and optoelectronic semi-conductor devices, as well as engineering applications.

A cornerstone of modern studies of sedimentary rocks is Uranium-Lead isotopic dating of zircon, a common mineral grain in sandstone. However, there is little understanding of how natural and induced bias can influence the outcome of dates. This project will focus specifically on quantifying: 1) the effect of crushing and grinding rocks prior to analysis, and 2) whether variation in hydraulic conditions during deposition influences zircon populations. The work of this project will lead to greater understanding of potential biases and lead to the development of mitigation strategies. This project will support partnerships between academia and government and engagement of pre-service and in-service teachers through a field experience around the Black Hills area in South Dakota.

The objectives of this proposal are to test the influence of 1) standard mineral separation processes for detrital zircon studies on grain size, grain shape, and surface microtextures, 2) hydraulic sorting after negating or accounting for the effects of mineral separation with high-n analyses (n=600). A major outcome of this work will be new protocols for provenance studies that integrates traditional, current, and new techniques to clearly document sediment sources and transport history. Our workplan includes: 1) tandem processing of samples using traditional and newer approaches followed by high throughput particle analysis to comprehensively characterize particle size and shape, and 2) single-grain, multi-parameter characterization of detrital zircons to explore for correlations between age groups, grain size/shape, surface morphology, and sediment transport history in a variety of ancient lithofacies representing different hydraulic conditions. This research will elucidate any inherent biases related to mineral separation and depositional energies in sedimentary systems. Being able to decipher the ‘true’ provenance in sedimentary basins will lead to better paleogeographic and sediment dispersal interpretations and tectonic reconstructions.

We are pleased to announce Heavy Mineral Analysis in Solving Tectonic Problems, a workshop led by Dr. Luca Caracciolo (FAU Erlangen-Nürnberg) and Dr. Sergio Andò (University of Milano Bicocca), who teach the European-based Heavy Mineral School at the University of Milano-Bicocca. This workshop will be organized to maximize the training of participants and allow for open discussions that focus on themes such as the effects of source rock lithologies, climate, hydraulic sorting, weathering, and lithification on detrital signatures, as well as single grain geochemistry and geochronology and future directions of research in these fields. The format of the workshop will include lectures, discussion sessions, and laboratory time on microscopes.

Total workshop participants will be limited to 30, with 20 spots reserved for Ph.D. students/post-docs, junior faculty (within 7 years of Ph.D.), and underrepresented groups (defined as the NSF (19-304) categories of Native American, Alaska Native, Black/African American, Hawaiian/Pacific Islander, Hispanic/Latino, or Asian).

Funding
Thanks to funding from the NSF Tectonics and Sedimentary Geology and Paleobiology programs, there is a nominal registration fee of $150, and limited funds are available to subsidize travel costs (prioritized for Ph.D. students/post-docs, junior faculty, and underrepresented groups). A block of rooms has been reserved at a special rate at the Iowa House Hotel (https://iowahousehotel.uiowa.edu/), located adjacent to the workshop building. Guests can book online using this link: Welcome! (innroad.com), and the promocode geologyworkshop to secure the discounted rate.

COVID-19 Precautions
The Earth & Environmental Sciences Department, in accordance with University of Iowa requirements and guidance, have protocols in place to permit faculty and students to access labs and continue research and teaching activities. In the event of a COVID-19 resurgence, we may adopt new protocols or reschedule the workshop.

Organizing Committee
Dr. Emily Finzel, University of Iowa
Dr. Bill McClelland, University of Iowa
Dr. David Peate, University of Iowa
Leon Aden, ExxonMobil retired and University of Iowa

The fluvial-to-marine transition zone (FMTZ) is the segment of a river that is intruded by marine currents and/or saline waters and is one of the most complex depositional environments on Earth. Although it is well-known that the FMTZ is an important feature of modern coastal systems, it has only been described from a few Holocene and ancient examples.  FMTZ deposits are economically important because a quarter of off-structure conventional petroleum traps in clastic reservoirs worldwide are hosted within FMTZ strata. In fact, one of the largest petroleum reserves in the world, the McMurray Oil Sands in Canada, is associated with FMTZ strata.  In complex stratigraphy such as these, an increased understanding of reservoir, seal, and source rock characteristics ultimately depends on improved partitioning and correlation techniques.  Therefore, a more complete comprehension of the stratigraphic record of FMTZs would not only aid in the evaluation of hydrocarbon reservoirs from these types of deposits but would also have implications for understanding coastal and wetland sedimentary and biotic evolution in response to climate change, and would permit more robust paleogeographic interpretations.  The objective of this proposal is to develop a litho-, chemo- and bio-stratigraphic framework for Lower-Middle Pennsylvanian strata in the Forest City basin, Iowa.  Our preliminary data suggest that these strata represent overall tidally-influenced fluvial aggradation in response to rising eustatic sea level.  Our goal is to test the combined approaches described above to determine if they will make it possible to interpret more detailed depositional histories in these economically valuable deposits.

The purpose of this grant was to purchase an X-ray Fluorescence (XRF) Spectrometer and accessories for use in classroom instruction, undergraduate and graduate research projects, and other scientific investigations.  This instrument lives in the UI MATFab Facility and allows bulk analysis of element concentrations in many environmental and geological materials. 

Various pieces of sample preparation equipment were purchased with this grant, including a Spex 8530 ShatterBox, a Spex 3636 X-Press, and a Katanax X-300 Electric Multi-position Fusion Fluxer.  The Spex 8530 ShatterBox is a ring and puck mill with a sound-proof enclosure that accommodates sample sizes ranging from 2 - 150 grams and is ideal for pulverizing dry, brittle samples and slurry grinding.  The Spex 3636 X-Press is a 35-ton hydraulic laboratory pellet press that is automatic and fully programmable and is designed for repetitive pressing of sample pellets for XRF, IR, and other analytical techniques.  The Katanax X-300 Electric Multi-position Fusion Fluxer prepares glass disks (beads) for XRF analysis and solutions for ICP/AA analysis.  All three pieces of sample prep equipment are operational and are housed in the EES department.

Though all continental mountain belts form as continental crust is shortened in regions of plate convergence, they exhibit major variations in width, elevation, and types of rocks involved. Given that many of these mountain belts are associated with some of Earth's most significant geological hazards and host prolific petroleum and mineral deposits, there is considerable societal and scientific interest in identifying the main factors responsible for their differences. This research will test a hypothesis that major differences in these mountain belts result from variations in the types and strengths of rocks that are present prior to the beginning of mountain building. This contribution is significant because it is expected to redefine the relative importance of these variations in influencing not only the geometry of mountain belts, but also the mechanisms through which they form. In addition to the scientific contributions of the project, important societal outcomes include workforce development through the education of graduate and undergraduate students in an important science, technology, engineering and mathematics (STEM) discipline. It fosters scientific literacy of the public through outreach efforts aimed at K-12 students that will result in the training and attraction of students to STEM careers. The project supports broadening of underrepresented groups in STEM. The project contributes to STEM educator development for high school teachers in the northern Rocky Mountain region through the implementation of a field-based course for high school science teachers in the northern Rockies. Results of the research will be disseminated through presentations at professional society meetings and peer-reviewed scientific literature.



The principal investigators will construct a structural, sedimentary, and thermochronologic record of the spatial and temporal transition in structural style--"thin-skinned" deformation of primarily sedimentary units or "thick-skinned" deformation of basement units--in east-central Idaho and southwestern Montana and compare it to the timing of "Laramide" flat subduction beneath Wyoming. The central hypothesis to be tested is that the transition in structural style along the proposed transect, which is outside the classic Laramide region, developed as a result of the pre-orogenic stratigraphic architecture of the upper plate. This work will provide a critical test of whether an intrinsic property of the upper plate, rather than a change in plate boundary geodynamics such as flat-slab subduction, can be a dominant mechanism for controlling orogenic structural style. The team will test the central hypothesis using 1) structural and stratigraphic investigations in east-central Idaho and southwestern Montana to constrain the spatial pattern of structural styles and coeval depocenters and 2) thermochronometry of thrust sheets to constrain the timing of exhumation related to thrusting and coeval foreland sedimentation. The research design includes zircon and apatite uranium-lead and (uranium-thorium)/helium dating, as well as apatite fission track thermochronometry in the context of field-based structural and stratigraphic investigations; these results will be used to build a regional kinematic model to evaluate the hypothesized non-flat-slab control on the spatio-temporal transition in structural style. Results from this work thus have the potential to transform our knowledge of the causal relationship between thin- and thick-skinned portions of continental fold-thrust belts and could warrant reevaluation of the importance of the pre-orogenic upper crustal architecture in affecting the first order geometries of modern and ancient continental mountain belts.

This award provides travel support for 25 field-camp leaders to be able to participate in Focusing the Lens on Field Safety: A Workshop for Field Trip Leaders that will take place at the University of Iowa from November 7-8, 2019. Field experiences are an important tool for training earth and environmental scientists, and most undergraduate programs require up to several weeks spent in the field to earn a degree. But field training is not without its risks, which are inherently different and have the potential to be more severe than classroom-based training. This workshop will help to guide field trip leaders towards a culture of improving field safety protocols by contributing to the development of a process that should ultimately become second nature and part of the normal preparations for field excursions. Part of the proposed workshop would be to develop materials for such a national repository. In addition, graduate students will be invited to join in the workshop and student participants will be given the opportunity to become ambassadors for field safety by being involved in the dissemination of information to other entities locally, statewide, and nationally. These opportunities will also allow graduate students to build their professional networks within and outside the academic community.

It is widely recognized that field-based training is pedagogically essential for integrating knowledge in the earth and environmental sciences, and it is crucial for employment in the hydrocarbon and environmental industries. The National Association of Geoscience Teachers (NAGT) and ExxonMobil have been leading the way in improving field safety protocol. However, one of the components that is underdeveloped in the current standards are protocols for field-camp-like scenarios where students are spread out over a wide area day after day. The outcome from this workshop will be a set of standard operating procedures (SOP) that can be adapted to any dispersed group field activity, whether an educational field trip, research field trip, or field camp. These procedures will help instructors mitigate accidents through pre-activity preparation and training that will result in fewer accidents and time lost in the field. Ultimately, this will enhance and improve scientific education because instructors will be confident in their safety protocols and able to focus on student learning, students will be assured that protocols are in place to ensure their safety, and a decrease in the number of incidents will lead to less disruption during limited and expensive field time.

The paradigm of convergent margins involves an oceanic plate of uniform character that subducts at a relatively steep (30 to 45 degrees) angle. Oceanic plates are not uniform, however, and instead consist of broad (plateaus) and narrow (aseismic ridges) regions of thickened crust, as well as spreading ridges where molten mantle material comes to the surface and creates new crust. An increase in crustal thickness or heat flow associated with these features causes the plates to be more buoyant than typical oceanic plates and to subduct at shallower or nearly flat angles. Flat-slab subduction of buoyant oceanic plateaus and aseismic and spreading ridges occurs along some of the world?s major convergent margins. The response of sedimentary basins above a these zones of flat slab subduction is not well understood. This project investigates the response of the Cook Inlet basin in south central Alaska to changes in subduction angle with special focus on the transition from steep subduction to flat subduction. A significant impact of the proposed research will be to provide a better understanding of the response of the upper plate and the sedimentary record to flat slab subduction, results which inform future studies of potential flat slab localities. The project will advance desired societal outcomes through: 1) full participation of women in STEM through support of two early career female PIs and outreach to minority high school students; 2) increased public scientific literacy and public engagement with science and technology through public outreach; and 3) development of a diverse, globally competitive STEM workforce though activities at Cincinnati high school, a short course for students and professionals in Anchorage, and training of graduate and undergraduate students.

The sequence, timing, and magnitude of upper plate processes related to flat-slab subduction are still poorly understood. Recognition of flat-slab subduction in the ancient geologic record is based primarily on changes in volcanic arc magmatism and less frequently on inboard migration of upper plate deformation, exhumation, and sedimentary basin evolution. However, very few studies have attempted to quantify the spatial and temporal patterns of surface uplift and erosion as it relates to subduction of a buoyant slab, and which result in changes to the types and locations of major sediment sources to adjacent basins. Here, a research team from the University of Iowa and University of Cincinnati will investigate the Late Cretaceous-Cenozoic sedimentary deposits of the Cook Inlet basin in south central Alaska, which constitute a complete sedimentary record of forearc basin strata deposited during three different modes of subduction. Normal subduction of oceanic crust in the Late Cretaceous was followed by flat-slab subduction of a spreading ridge (ca. 62-50 Ma), and later by subduction of an oceanic plateau (ca. 40-0 Ma). Two hypotheses are tested: (1) subduction of a spreading-ridge in late Paleocene?Eocene time resulted in a change in topography and a shift in sediment sources from the adjacent arc to the retroarc region; (2) flat-slab subduction of an oceanic plateau from Oligocene time to the present resulted in the creation of topography above the flat-slab region and an overall contraction of the forearc basin drainage area. The researchers investigate the sedimentary record of flat slab subduction in southern Alaska by examining Late Mesozoic and Cenozoic strata in core samples from the Cook Inlet forearc basin through traditional stratigraphic analysis combined with provenance and thermochronologic techniques of double-dating detrital zircons using uranium-lead and fission track dating, rare earth element analyses of mudstones, and sandstone petrography. The integration of these datasets enables the investigation of the patterns of regional exhumation, magmatism, and sediment transport across the area and interpretation these patterns in terms of regional tectonics and changes through time. This study will provide a foundation for new tectonic and provenance models of forearc basins that have been modified by flat-slab subduction processes.

Kapolas, A., Finzel, E.S., Horkley, L.K., and Peate, D.W. (in press). Whole-rock geochemical provenance, Cook Inlet forearc basin, south-central Alaska, USA. GSA Bulletin, doi.org/10.1130/B37418.1.

Finzel, E.S., Rosenblume, J.A., Pearson, D.M., and Zippe, P.A. (2023). Timing of the Transition from Sevier- to Laramide-Style Tectonism in Southwestern Montana Based on the Provenance of the Frontier Formation, North American Cordillera. Tectonics, doi.org/10.1029/2023TC007777.

Gardner, C.T., Finzel, E.S., Pearson, D.M., and Rosenblume, J.A., in press, Foreland basin response to middle Cretaceous thrust belt evolution, southwestern Montana, USA: Geosphere.

Parker, S., Pearson, D., and Finzel, E., in press, A Thermal Profile Across the Idaho-Montana Fold-Thrust Belt Reveals a Low-Relief Orogenic Wedge That Developed Atop a Pre-Orogenic Basement High: Lithosphere.

Rosenblume, J.A., Finzel, E.S., Pearson, D.M., Gardner, C.T., 2022, Middle Albian provenance, sediment dispersal and foreland basin dynamics in southwestern Montana, North American Cordillera: Basin Research, https://doi.org/10.1111/bre.12645.

Schwarz, E., Finzel, E.S., Veiga, G.D., Rapela, C.W., Echevarria, C., and Spalletti, L.A., 2020, U-Pb geochronology and paleogeography of the Valanginian–Hauterivian Neuquén Basin: Implications for Gondwana-scale source areas: Geosphere, v. 17, p. 1–27, https://doi.org/10.1130/GES02284.1.

Finzel, E.S. and Rosenblume, J.A., 2020, Dating lacustrine carbonate strata with detrital zircon U-Pb geochronology. Geology doi: https://doi.org/10.1130/G48070.1

Rosenblume, J.A., Finzel, E.S., Pearson, D.M., 2021, Early Cretaceous provenance, sediment dispersal, and foreland basin development in southwestern Montana, North American Cordillera: Tectonics, doi.org/10.1029/2020TC006561

Thomas, W.A., Gehrels, G.., Sundell., K.E., Greb, S.F., Finzel, E.S., Clark, R.J., Malone, D.H., Hampton, B.A., Romero, M.C., 2020, Detrital zircons and sediment dispersal in the eastern Midcontinent of North America, Geosphere (2020) 16 (3): 817–843.

Garber, K.L., Finzel, E.S., Pearson, D.M., 2020, Provenance of syn-orogenic foreland basin strata in southwestern Montana requires revision of existing models for Laramide tectonism: North American Cordillera: Tectonics.

Finzel, E.S., 2019, Partitioning pervasive detrital geochronologic age distributions in the southern Alaskan forearc: Frontiers in Earth Science.

Enkelmann, E., Sanchez Lohff, S., Finzel, E.S., 2019, Detrital zircon double-dating of forearc basin strata reveals magmatic, exhumational, and thermal history of sediment source areas: GSA Bulletin.

Enkelmann, E., Finzel, E.S., Arkle, J., 2019, Deformation at the eastern margin of the Northern Canadian Cordillera: potentially related to opening of the North Atlantic : Terra Nova.

Reid, M., Finzel, E.S., Enkelmann, E., and McClelland, W.C., 2018, Detrital zircon provenance of Upper Jurassic–Upper Cretaceous forearc basin strata on the Insular terranes, south-central Alaska, in Ingersoll, R.V., Lawton, T.F., and Graham, S.A., eds., Tectonics, Sedimentary Basins, and Provenance: A Celebration of William R. Dickinson’s Career: Geological Society of America Special Paper 540, p. 1–20, https://doi.org/10.1130/2018.2540(25).

Kissock, J.K., Finzel, E.S., Malone, D., Craddock, J., 2018, Lower-Middle Pennsylvanian strata in the North American midcontinent record the interplay between erosional unroofing of the Appalachians and eustatic sea level rise: Geosphere.

Finzel, E.S., 2017, Detrital zircon microtextures and U-Pb geochronology of Upper Jurassic to Paleocene strata in the distal North American Cordillera foreland basin: Tectonics.

Finzel, E.S. and Enkelmann, E., 2017, Miocene-Recent sediment flux in the south-central Alaskan forearc basin governed by flat-slab subduction: G-cubed.

Finzel, E.S. and Ridgway, K.D., 2017, Links between sedimentary basin development and Pacific basin plate kinematics recorded in Jurassic to Miocene strata on the western Alaska Peninsula: Lithosphere.

Finzel, E.S., Enkelmann, E., Falkowski, S., Hedeen, T., 2016, Long-term forearc basin evolution in response to changing subduction styles in southern Alaska: Tectonics, v. 35, doi:10.1002/2016TC004171.

Finzel, E.S., Flesch, L.M., Ridgway, K.D., Holt, W.E., and Ghosh, A. , 2015, Surface motions and intraplate continental deformation in Alaska driven by mantle flow: Geophysical Research Letters, v. 42, p. 4350–4358.

Finzel, E.S., Ridgway, K.D., and Trop, J.T., 2015, Provenance record of Cenozoic forearc basins in southern Alaska: detrital zircon signatures of flat-slab subduction: Geosphere, v. 11, p. 823-849.

Finzel, E.S., 2014, Detrital zircons from mid-continent strata reveal an Appalachians-Western Interior Basin connection: Lithosphere, v. 6, p. 378-382.

Finzel, E.S., Flesch, L.M., and Ridgway, K.D., 2014, Present-day geodynamics of the northern North American Cordillera: Earth & Planetary Science Letters, v. 404, p. 111-123.

Ridgway, K. D., Trop, J. M., & Finzel, E.S., 2012, Modification of continental forearc basins by spreading ridge subduction and flat-slab subduction processes: A case study from southern Alaska in Busby, C., and Azor, A. (Eds.), Tectonics of Sedimentary Basins: Recent Advances. Wiley-Blackwell.

Finzel, E.S., Flesch, L. M., & Ridgway, K. D., 2011, Kinematics of a diffuse North America-Pacific-Bering plate boundary in Alaska and western Canada: Geology, v. 39, p. 835-838.

Finzel, E.S., Trop, J. M., Ridgway, K. D., & Enkelmann, E., 2011, Upper plate proxies for flat-slab subduction processes in southern Alaska: Earth & Planetary Science Letters, v. 303, p. 348-360.

Finzel, E.S., Ridgway, K. D., Reifenstuhl, R. R., Blodgett, R. B., White, J. M., & Decker, P. D., 2009, Stratigraphic framework and estuarine depositional environments of the Miocene Bear Lake Formation, Bristol Bay basin, Alaska: Reservoir strata in a frontier gas-rich basin: American Association of Petroleum Geologists Bulletin, v. 93, p. 379-405.