USING BIOSTRATIGRAPHY TO INTERPRET SEA-LEVEL VARIATIONS - A STUDENT EXERCISE

Fossils are the key to dating the past, but geoscience students are rarely exposed directly to their use in conversion to geologic time and deciphering regional sediment history. Therefore, we have developed a software package and a student exercise that uses real-data sets of fossils and sediment trends from drill sites in the Gulf of Mexico to illustrate how one applies biostratigraphy to understand regional depositional history and sea-level variations.

Undergraduate students in computer engineering at Purdue developed a crossplot module for the TimeScaleCreator visualization system for Earth history, and selected offshore and onshore well datasets spanning the Miocene and Oligocene epochs along an approximate transect were contributed by PaleoData, by the Bureau of Offshore Energy Management and by the Gulf Basin Depositional Synthesis Project (UT Austin). Crossplot tool enables the comparison of ranges of microfossils in the drill sites to regional Gulf of Mexico biostratigraphy. The depth-to-age conversion routine enables the students to convert the accompanying sediment facies to the geologic time scale displayed on-screen as a time-adjusted transect. After rescaling the wells from depth to timescale, one can identify large-scale packages of sand-dominated influxes separated by more condensed clay-dominated episodes. These packages correspond to named widespread depositional sequences caused by a combination of major sea-level variations (“sequence stratigraphy”) that modulated the distal tectonic-induced changes in the North America drainage systems.

The module includes an introduction to biostratigraphy and general depositional history of Gulf of Mexico, and a detailed walk-through of how to convert a well from depth to a geologic time scale using correlations of microfossils and nannofossils. The remaining wells are more quickly converted using embedded look-up functions in TimeScale Creator, so that the students can concentrate on testing hypotheses of potential hiatuses and preservation artifacts, and interpreting the depositional episodes for variations in sea level. An illustrated teacher’s manual provides our suggested solutions. The suite and free software is available from the TimeScale Creator website (www.tscreator.org).