Charlotte J. Mehrtens

Polygonal faults in chalk: Insights from extensive exposures of the Khoman Formation, Western Desert, Egypt

Although polygonal fault systems and related features are common in fine-grained sediments in modern submarine basins and have been studied in basins worldwide using three-dimensional (3-D) seismic data, extensive on-land exposures have remained elusive. We report here on the discovery of a polygonal fault system occurring in nearly continuous surface exposure over ∼900 km 2 in chalk of the Cretaceous Khoman Formation near Farafra Oasis, Egypt. Field exposures reveal polygon boundaries defined by clusters of dozens of normal faults with strongly grooved fault surfaces and coarse calcite veins along faults with evidence for multiple fluid flow events. Geometric patterns and fault intersections reveal that mechanically interacting normal faults with multiple orientations were active contemporaneously in a horizontal strain field that was essentially isotropic and extensional. We interpret the very steep dips (∼80°) to reflect fault initiation in response to elevated pore fluid pressures. In the uppermost part of the Khoman Formation, a terrain of isolated circular structures displaying shallow inward dips overlies the polygonal fault network. The spatial relationship to the underlying faults is consistent with these small circular basins having formed as fluid escape structures as the polygonal fault system evolved. Outcrops in the Khoman Formation provide an unprecedented look into the 3-D geometry of a polygonal fault system, providing context for the analysis of analogous systems in marine basins and other on-land exposures.

Reef morphology and sediment attributes, Roatan, Bay Islands, Honduras

A reef monitoring program off Roatan, Bay islands, Honduras has produced base line data for platform bathymetry, major macrofauna distribution, and sediment attributes. Because erosion accompanying accelerated island development will be increasing in the near future, measurements of total suspended solids and sedimentation rate were made. Results show TSS range from 8 to 70 mg/l in the summer and 30 to 222mg/l in the winter. Sedimentation rates in the fore reef, where finer-grained terrigenous material would potentially accumulate, are 0.14mg/cm2/day to 7.07mg/cm2/day in the summer and 41 to 71mg/cm2/day in the winter. The summer values are well within the ranges of published results from other Caribbean carbonate platforms, however winter values are up to ten times that of other localities.

An Introductory-Level Field-Based Course in Geology and Botany

At the University of Vermont (UVM), we offer a year-long, interdisciplinary, residentially based course for first-year students interested in the natural sciences. This course, entitled “The Geology and Ecology of Lake Champlain,” takes advantage of the Universitys location between Lake Champlain and the Green Mountains. Although the University offers students introductory-level courses in both geology and ecology, these are high-enrollment lecture-format classes with some field trips. “The Geology and Ecology of Lake Champlain” offers a small group of students a field-based course that seeks to integrate these related disciplines. In the course, the students become active hands-on participants in small research projects. Although UVMs geographic location has certain unique features, we believe that the fundamental approach of this course can be adopted by universities in other locales.