Spatial Learning Challenges Associated with Sequence Stratigraphy

Sequence stratigraphy combines concepts from sedimentology and stratigraphy to understand the order of depositional events based on rock characteristics. Sequence diagrams focus on either the spatial or temporal relationship between depositional events. For example, Figure 1 shows a stratigraphic sequence of sedimentary deposits that one might find in a delta, where a river meets a larger body of water. Geologists try to interpret these formations from seismic data, particularly to discover off-shore oil and gas deposits. The combination of required content knowledge, spatial visualization skills, and temporal reasoning skills makes sequence stratigraphy a complicated topic for students and fertile ground for researchers (Herrera and Riggs, 2013).

Figure 1. Sequence of sedimentary deposits shown in a cross-section, or vertical slice through a delta (SEPM STRATA, 2015).

Herrera and Riggs (2013) identified four foundational concepts associated with sequence stratigraphy in a qualitative study with 25 undergraduates and 2 graduate geology students enrolled in a 300/400 level sedimentology and stratigraphy course. The study categorized students' conceptions about: base level, eustasy or global sea level, relative sea level, and accommodation space. Base level is defined as the surface of equilibrium between deposition and erosion (Catuneanu, 2006). More generally, base level is referenced to a local elevation to which erosion will persist. This level is typically below the water level of a lake or sea as it will include the level of wave erosion. Figure 2 shows how a lake and ocean can change the path of base level for river erosion. Base level is difficult because many students claim to have no recollection of the concept. Herrera and Riggs (2013) found that of the undergraduate geology seniors studied, 40% (approximately 6 of the 16 seniors) had never heard of base level. Furthermore, few advanced undergraduate or graduate students related base level to sea level. Students who held scientific conceptions recognized that base level is different from sea level, but related to: sea level, the level of erosion for rivers, and the space available for deposition of sediments (i.e. accommodation space).

Figure 2: River profile with lake, illustrating the concept of local base level is compared to a river profile with sea level as the local base level (SEPM STRATA, 2015)

The second concept is eustasy, also referred to as global sea level. Herrera and Riggs (2013) define eustasy as a "worldwide sea level measured between the sea surface and a stationary- datum at the center of the Earth" from Burton et al. (1987). Eustasy may be thought of as a globally referenced sea level, regardless of whether a particular region sits at higher or lower elevation. Figure 3 compares how eustasy, relative sea level, and water level are measured from different points in the Earth. Students' understanding of eustasy ranged from alternative conceptions, where students understood little beyond eustasy having to do with global changes, to incomplete scientific conceptions. In the latter cases students explained some aspects of the temporal and spatial scales that play a role in eustasy and explained what causes large-scale changes in sea level. Though, some students understood that this is a complex system with many moving parts, no students recognized that eustasy was measured from the datum at the center of the earth, a central part of the definition.

Figure 3. Shows a comparision of where eustasy, relative sea level, and water level are measured from. The orage layer represents solid bedrock (SEMP, 2015)

The third foundational concept, relative sea level, is any change in sea level relative to a local datum point (Catuneanu, 2002). For example, if a region experienced tectonic uplift, the relative sea level would be the elevation difference between the surface of the sea and the bedrock surface. The local datum, as shown in the figure above, is determined based on the elevation of the local bedrock, not including the loose sediment above the bedrock (Catuneanu, 2006). Herrera and Riggs (2013) found that 30% of the students (N=8) in their study thought of relative sea level as a spatial concept, while others related it to changes over time, such as tides. As with eustasy/global sea level, no students related relative sea level to the concept to the local datum.

The fourth concept, accommodation space, is defined by Herrera and Riggs (2013) as the "space available for sediment to accumulate. It is influenced by tectonic subsidence and uplift and/or sediment supply" based on Coe et al. (2005). Accommodation space is also influenced by changes in sea level and climatic conditions. As the lithosphere subsides or sea level rises, more space is available to accommodate sediment. As the lithosphere is uplifted or sea level falls, less space is available to accommodate sediment. Herrera and Riggs (2013) analyzed students' understanding of accommodation through their ability to combine influencing factors. Some students identified only one factor, transport mechanism, while others were able to identify five or more influencing factors. More than half the students identified three or less factors. Most students knew they needed a basin or space for sediment to collect in addition to sediment supply or transport but few connected other factors that determine accommodation space.

Herrera and Riggs' (2013) study shows us that even advanced geology majors have a range of conceptions about sequence stratigraphy. A cursory review of diagrams on well-respected websites and in textbooks reveal that the four concepts listed above are conceptually and spatially challenging. Base level, in particular, is visually misrepresented or not well related to the other three concepts: eustacy, relative sea level, and accommodation (Figure 3). Some figures do a better job of representing that base level will cause erosion to local areas (Figure 2) but still inappropriate equate sea level to a global base level, when really it should be equated to relative sea level, not eustacy.

This leads us to consider what spatial thinking skills might be needed to understand these four foundational ideas. Most generally, students are being asked to engage in retrodiction (Ault, 1998). Students are given existing diagrams of past sedimentation and are asked to make interpretations about where sea level (relative and global) and base level use to be and how they have changed over time. Retrodiction is not unique to these concepts but it is conceptually challenging if students do not understand the fundamental processes that formed the features over time. If students struggle with the basic concepts identified by Herrera and Riggs (2013), then how can they unravel the geologic history of existing depositional sequences? A second challenge embedded in the retrodiction is the dynamic three- and four- dimensional (3-D and 4-D) nature of stratigraphic problems.

All four of the concepts describe 3-D features, but they represent changes to a depositional structure over time, a fourth dimension. To unpack the spatial skills that may be required to decipher a stratigraphic section, we categorized the concepts using the Newcombe and Shipley (2012) framework. Extrinsic- Dynamic spatial tasks involve "transforming the inter-relations of objects as one or more of them moves". Within the Extrinsic-Dynamic category there are three types of visual skills that students must use to be able to make these 4-D interpretations (Newcombe and Shipley, 2012). The first is perspective taking, or looking at the scene from a different vantage point (Newcombe and Shipley, 2012). In geology courses, students are given a cross-section (2-dimensional view) of a stratigraphic sequence, but are still expected to infer the overhead view of a river entering the ocean (3^rd dimension) and make interpretations that incorporate processes occurring over both views.

The second Extrinsic-Dynamic spatial skill is relations among objects (Newcombe and Shipley, 2012). This can be simple, the relationship between the ocean bedrock and surface of the sea (i.e. relative sea level), or more complicated. In sequence stratigraphy (Figure 1) need to evaluate the sediment source, uplift or subsidence, and relative sea level to understand the relationship of each sequence (outlined in red in Figure 1) to another sequence.

The third Extrinsic-Dynamic spatial skill is updating movement through space (Newcombe and Shipley, 2012). Each of the four sequence stratigraphy concepts (i.e. eustasy, relative sea level, base level, and accommodation space) can be changing relative to one another at any given time. Of the four, eustasy, or the change in sea level from the center of the earth, typically changes over the longest time scales and therefore has the least impact on deciphering stratigraphic sequences. Relative sea level and accommodation space both relate to local changes in sea level relative to the local bedrock. This includes elevation changes due to uplift or subsidence (tectonic movement that raises or lowers the crust at a specific area). Accommodation is influenced by sediment supply combined with subsidence or higher relative sea level. Therefore, these two concepts have a strong spatial link. Base level is influenced by the same tectonic processes as relative sea level and accommodation, but has a local component. Changes in the elevation of lakes determine erosion rates of rivers, which can slow the process of erosion to relative sea level.

Our preliminary categorization of spatial skills involved in understanding sequence stratigraphy is necessarily insufficient. Conceptual understanding of the fundamental processes is a highly complex undertaking that appears to be poorly taught on the undergraduate level based on Herrera and Riggs (2013) study. Yet, geologists in industry wager high gambles on being able to pinpoint the location of oil and gas reserves in these complex sequences. We pose the following questions to our blog readers:

· Do your students struggle with these ideas?

· Do you struggle to communicate these ideas to students?

· In a perfect world, where do we start trying to help students understand these spatial complexities? In introductory courses? In stratigraphy classes?

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