Reading: Boggs Chapter 12
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 which is 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 from Lab 8, text Figure 12.14, and handout (Boggs, 1995 Fig. 14.8).
Regression can be caused by: (1) caused by lowering of sea level, as we might expect (= "forced" regression), or (2) it can with stationary or rising sea leve due to progradation of sediment into a standing body of water (as a delta might do). In the second case, the effect of sediment input is so strong that it overwhelms the efect of stationary or rising sea level. This leads us into concepts of Sequence Stratigraphy (see below).
Reading: Boggs Chapter 13 (section 13.3), and the Online Guide to Sequence Stratigraphy by Steven Holland, University of Georgia (big thanks to him for that). And, Sequence Stratigraphy at the University of South Carolina, an in-depth treatment.
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.
We also discussed "forced" regression versus "normal" regression (see class handout), and watched a movie from the Univ. South Carolina web page to help undesrand the geometries and controls on sequence stratigraphy.
Use of Sequence Stratigraphy in Sea-Level Analysis. Boggs Figure 13.8 shows 3-step analysis:
Although it has been successful in many ways, and is a powerful approach to interpreting stratigraphy, there has been alot 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 eustatic sea level. This assumption ignores the effects of variable subsidence or uplift due to tectonic forces, and varying sediment input through time.