What’s new in exploration

 Vol. 233 No. 6

Earth history is recorded at many scales. Trees accumulate decades, centuries, even millennia of layered rings reflecting climate variations of time past. Sediments are laid down over eras and epochs, sometimes carried to the deepest parts of the ocean and immediately buried. Paleontologists have organized plant and animal fossils to date many formations.

Macrostratigraphy is an emerging technique used by geologists to tie together data from disparate regions. It’s in the limelight now, with a recent letter published in Nature, and an upcoming paper in the GSA Bulletin. Both use data synthesized within the Macrostrat project at the University of Wisconsin-Madison.

Alan R. Carroll, professor of geology at the UW-Madison explains, “The basic technique is to treat individual stratigraphic units as if they were fossils, by documenting the geologic timing of their origination and termination (precisely analogous to first and last appearances of fossil taxa).  From there, you can add up the total number of units that originate or terminate within a given time interval, as well as total stratigraphic diversity. The net result is a comprehensive record of how sedimentation has changed through time that can be used to quantitatively test hypotheses concerning the controls on sedimentation.”

Shanan E. Peters, an assistant professor in the Department of Geology and Geophysics at UW-Madison, is looking at physical environmental changes encoded in the sedimentary rock record. He’s interested in macrostratigraphy, because “sedimentary rocks provide an important spatial and temporal framework for understanding environmental change and the processes of extinction and evolution.”

Peters and Robert R. Gaines, Pomona College, recently published a macrostratigraphic analysis of the great unconformity, a globally widespread stratigraphic surface, which separates continental crystalline basement rock from Cambrian shallow marine sedimentary rocks.¹

They used new stratigraphic and geochemical data to show that early Paleozoic marine sediments, deposited 480–540 million years ago, record both an expansion in the area of shallow epicontinental seas and anomalous patterns of chemical sedimentation that indicate increased oceanic alkalinity and enhanced chemical weathering of continental crust.

Comprehensive database. In addition to field work globally, Peters pulls geological data for large areas from published studies and weaves it together in Macrostrat, a comprehensive relational database that is powered by open-source software. “A lot of really great field work has already been done,” Peters said. Macrostrat was created to “leverage modern database and internet technology and to capitalize on a large, initial dataset.” Viewing the rock record in a new way makes data more useful for teaching and research.

Macrostrat Beta 0.3 is the initial interface for the database. It’s “intended to become a community-driven platform for macrostratigraphy, which facilitates the rigorous testing of hypotheses related to the spatial and temporal distribution of sedimentary, igneous and metamorphic rocks, and proxy data extracted from them.” Lithostratigraphic units from individual geographic locations (columns) are the “fundamental currency” of Macrostrat, and serve as the hub for linking data types.

Macrostrat is synced daily with the paleobiology database. Peters’ team has matched more than 91% of all North America PaleoDB collections to lithostratigraphic units, using a combination of stratigraphic nomenclature and geography. Integrating rock and fossil records will allow quantitative analysis of their covariation (where it exists). The team plans to add online analytical tools, geochemical and measured section data.

Macrostrat coverage. Macrostrat contains data from more than 33,965 rock units from 1,475 geographic regions, and nearly 1,500 radioisotopic ages.

North American records (23,843 units, 6,977 packages, 949 columns) constitute about 70% of the data. The data are drawn from the Geological Survey of Canada, which published a series of four, country-wide correlation charts in the 1970s, and from COSUNA project data (Correlation of Stratigraphic Units of North America), published by AAPG in the late 1980s and early 1990s. Macrostrat ver. 0.3 also incorporates the database of the GSA’s 2005 geological map of North America.

Other areas of concentration include New Zealand and the deep sea. The offshore data span the Pacific, North and South Atlantic, and Indian Oceans. Macrostrat also includes the circum-Caribbean basin, derived from correlation charts published in GSA’s Decade of North America Geology.

Exploration. Carroll suggests the most obvious petroleum application will be in global resource assessment, “looking for temporal patterns in the occurrence of hydrocarbon-bearing rocks.” He believes macrostratigraphic analysis would be “very effective for predicting the total resource likely to be available from shale gas and oil reservoirs ...”

Carroll and colleagues from UW-Madison, Sonoma State University, and Thailand’s Kasetsart University have used this approach at the basin scale to examine how patterns of oil shale deposition in Wyoming’s Green River formation are controlled by Milankovitch periodicity.

They use macrostratigraphy to derive facies-specific time series, integrating stratigraphic patterns from localities across the basin, incorporating “spatial variability directly into quantitative stratigraphic analyses.”² It’s an example of what’s possible, using the abundant data available.  

1. Peters, S. E. and R. R. Gaines. Apr. 18, 2012, “Formation of the ‘Great Unconformity’ as a trigger for the Cambrian explosion.” Nature, vol. 484, pp. 363-366. Link: http://www.nature.com/nature/journal/v484/n7394/full/nature10969.html
2. Aswasereelert, W., et al., in press, “Basin-scale cyclostratigraphy of the Green River formation, Wyoming.” GSA Bulletin.