Reading: Boggs Chapter 12, and Prothero Handout
Walther's Law. "Facies that occur in a conformable vertical sucession were produced by deposition in in laterally adjacent, contiguous depositional environments".
In order to produce this conformable vertical succession of facies, the environments must have shifted laterally through time, with continuious sedimentation. This allows us to convert a vertical straigraphic section into a reconstructed map of original depositional environments, very powerful concept. It's important to understand where we can or cannot apply Walther's Law, or else we may apply it incorrectly and arrive at an unsupportable intepretation. See text Figure 12.13.
Transgression and Regression. Defined as lateral migration of the shoreline toward (transgression) or away from (regression) the center of a land mass, usually a continental interior.
Main Controls on Trangression and Regression:
(1) Eustatic Sea Level rise or fall
(2) Tectonic subsidence or uplift
(3) Rate of sediment supply (influx): high or low (fast or slow)
Factors 1 and 2 combine to produce what we call "relative sea level". To successfully interpret stratigraphy, we have to consider the dynamic balance between: (a) rate of relative sea level rise or fall, and (b) rate of sediment influx. These rates and their relative magnitude vary through time to create the stratigraphic record. See figures in the class handout and text Figure 12.14.
We recognize two types of Regression: (1) "forced regression", which is forced by lowering of relative sea level, or (2) "normal regression", which results from progradation of sediment into a standing body of water during stationary or rising relative sea level, as deltas often do. In some cases very high rates of sediment input can overwhelm the effect of stationary or rising sea level.
Reading: Boggs Chapter 13 (13.1-13.3), and the Online Guide to Sequence Stratigraphy by Steven Holland, University of Georgia (big thanks to him for that). And Sequence Stratigraphy II - at SEPM (Society for Sedimentary Geology).
A Depositional Sequence is a relatively conformable succession of genetically related strata bounded by unconformities or their correlative conformities. So, sequence boundaries are unconformities, which are defined here (narrow definition) as surfaces produced by subaerial exposure and erosion. Thus, every sequence records one cycle of relative sea-level fall, rise, and fall.
Sequences are commonly divided into different "systems tracts", including low-stand, transgressive, and highstand systems tracts. They occur in distinctive parts of each sequence and, together, record cycles of relative sea-level rise and fall.
Parasequences. Sequences are made up of smaller, individual shallowing- and coarsening-up packages called parasequences, which are internally conformable and are bounded by marine flooding surfaces. We can say that Walther's Law applies within parasequences but not across marine flooding surfaces that bound them. Most parasequences probably record gradual progradation of sediment from a shoreline (often deltaic), followed by abandonment and submergence when sediment input stops due to channel or delta-lobe switching.
Groups, or sets of parasequences may be arranged in a varity of different geometries that record the evolution of relative sea level and sediment influx during the cycle through which a sequence evolves. These different geometries are referred to as parasequence stacking patterns, which may be progradational, aggradational, or retrogradational, depending on what's happening with the long-term balance between relative sea level and sediment influx over the course of multiple parasequence cycles.
Use of Sequence Stratigraphy in Sea-Level Analysis. Boggs Figure 13.18 shows 3-step analysis:
Although it has been successful in many ways, and is a powerful approach to interpreting stratigraphy, there has been quite a lot of controversy over this type of analysis. The main problem was that the Exxon group claimed their curves record rises and falls in global eustatic sea level through time (Figs. 13.20, 13.21), which assumes that all changes in relative sea level were caused by changes in global eustatic sea level. This assumption ignores the possible (and very real) effects of variable subsidence or uplift due to tectonic forces.
We will also watch a movie from the Univ. South Carolina web page to help understand the geometries and controls on sequence stratigraphy.