Academic journal article Journal of Geoscience Education

Using a Modified Hele-Shaw Cell to Understand the Layering of Strata

Academic journal article Journal of Geoscience Education

Using a Modified Hele-Shaw Cell to Understand the Layering of Strata

Article excerpt

ABSTRACT

The modified Hele-Shaw cell lets us simulate sedimentation in a wide variety of situations from a river bed to a sedimentary basin. It illustrates deposition and erosion, stratigraphy, progradation, aggradation, and the origin of sedimentary structures such as beds, laminations, ripples and slumps. Using a narrow acrylic tank based on the Hele-Shaw cell, sediment and water are added at one end and water is withdrawn at the other. As observers watch sedimentary processes-in real-time, sediments build up in layers in the tank, demonstrating the nature of strata as a geological record. This engaging experiment is suitable for primary to university education. It has principally been used to teach the fundamentals, but can also be used by advanced students to experiment with other sediment-water interactions such as groundwater flow and angle of repose.

INTRODUCTION

Instructors tend to think that the origin, description, and classification of sediments and sedimentary rocks are easily understood when compared to teaching the origin of igneous and metamorphic rocks. This is because we can conduct field trips to modern sedimentary environments, whereas this is difficult, if not impossible, for the other two rock types. Accordingly, we (the authors) lead many field trips to the beaches and small rivers of Nova Scotia to explore modern sedimentary processes and look at coastal exposures of sedimentary rocks so that students can compare modern and ancient sedimentary environments.

However, even field trips to modern sedimentary environments can be problematic in that they usually involve making observations of sedimentary structures such as ripples, mega-ripples, desiccation cracks, current lineations, slumps, etc. on a usually very active, present-day quasi-2-dimensional horizontal bedding plane. Sometimes trenches may be dug to observe sedimentary features in section (such as beading, graded beds, cross-beds, stratification, etc.), but trenches are usually unstable and collapse. As well trenches are small and provide a restricted view, unless you have strong and willing participants, lots of time, no regard to the environment, and good light. It is quite a challenge to demonstrate the ways in which sedimentary features are formed and preserved in the rock record.

Compounded with these problems it is even more difficult to make the leap from modern sediments and structures to those seen in sedimentary rocks. Most features seen on the beaches and in the streams are observed on the 2-dimensional horizontal planar surface, whereas many of the features seen in the outcrop are in section. Students have no trouble seeing and understanding how ripple marks form on a modern beach but when they see ripple marks exposed in an adjacent cliff section they erroneously conclude that the ripples are a modern feature. They have difficulty understanding how ripples can be preserved in the rock record and how the strata build up over geological time.

One way we are trying to overcome the above problems is by having the students experiment with sediment and water in an acrylic box very similar to an ant farm and formally known as a modified Hele-Shaw cell (Hele-Shaw, H.S. 1898, Schlumberger Limited, 2006). As this is the first exercise in a series of laboratories dealing with sediments, it can be referred to many times in future classes when dealing with structures seen at the scale of grains and/or outcrops, or when talking about sedimentary basins. It allows students to observe deposition and erosion, and the results of those processes such as sedimentary structures (bedding, graded beds, ripples, slumps, etc.), faciès changes and correlation of beds.

Doing an experiment where students can watch sedimentary layers develop with many of the resultant complexities makes layering in the field easier to understand, whether in rocks or modern sediments. The experiment shows students how a model in the laboratory can replicate natural processes, be tested by comparison with naturally formed features, and provide controls or limits on our interpretation of sedimentary rocks. …

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