= coastal plain (rivers + floodplain) = tidal marsh = lagoon Exercise 2. This exercise is an application of Walther’s Law and lithostratigraphic correlation. You will use 5 maps that define the extents of 5 depositional environments through time. Time 5 defines the position of the coastal system in the mid-Holocene, Time 4 represents the late mid-Holocene, Time 3 represents the early late Holocene, Time 2 represents the late Holocene, and Time 1 represents the present-day shoreline. The five depositional environments are listed in the legend below. Walther’s Law states that the vertical succession of depositional facies reflects lateral changes in depositional environment = coastal plain (rivers + floodplain) = tidal marsh = lagoon = barrier island complex = continental shelf (open marine)
Exercise 2, continued.
Exercise 2, continued.
Exercise 2, continued.
Exercise 2, continued.
Exercise 2, continued.
Work flow: 1. Place tick marks on each time line defining the position of every facies boundary that intersects the B’ – B transect. 2. Once tick marks are places on all 5 time lines apply Walther’s Law and the principles of lithostratigraphic correlation to connect the deposits associated with the 5 environments between Time 5 and Time 1. 3. Describe the history of coastal progradation and retrogradation observed in this cross section. Exercise 2, continued. B’ B Time 2 Time 1 (youngest) Time 3 Time 4 Time 5 (oldest) 10 m 1000 m Scale:
A A’ Exercise 2, continued. Work flow: 1. Place tick marks on each time line defining the position of every facies boundary that intersects the A – A’ transect. 2. Once tick marks are places on all 5 time lines apply Walther’s Law and the principles of lithostratigraphic correlation to connect the deposits associated with the 5 environments between Time 5 and Time 1. 10 m 1000 m Scale: A A’ Time 2 Time 1 (youngest) Time 3 Time 4 Time 5 (oldest)
Lithostratigraphic Interpretation Exercise 3. This exercise is designed to illustrate the differences between stratal correlations based on lithology and those based on interpreted parasequences. Provided are Gamma Ray (GR) and lithology logs for 5 hypothetical wells. Sandstone and coal usually has low GR signatures (curves trends toward the left), whereas mudstone usually has a high GR log signature (the curve trends to the right). Lithostratigraphic correlation: Correlate logs V-Z in the section below using lines to connect the tops and bases of rock units based solely on lithologic and log-pattern similarities. #1 and the two arrows define the position of Timeline 1 that will be used in the second half of this exercise. #1 Lithostratigraphic Interpretation
Parasequence Interpretation Exercise 3, continued. Parasequence correlation: (1) The first step in parasequence correlation is to identify and correlate the flooding surfaces at the tops of all 5 coarsening/shallowing-upward successions; it is often to identify these surfaces in wholly continental successions. (2) Correlate the 5 facies between the five wells keeping in mind that lithofacies can pinch out laterally as the shoreline systems prograde and/or retrograde. Parasequence correlation is significantly different from lithostratigraphic correlation because it allows for abrupt termination/onset of depositional facies. These abrupt changes in facies are a consequence of abrupt shifts in depositional environments (e.g., channel avulsion) versus progressive lateral movement of depositional environments (e.g., Exercise 2). #1 Parasequence Interpretation
Exercise 3, continued. Draw simple plan-view paleogeographic maps for your lithostratigraphic and parasequence correlations along the timeline labelled #1 in each cross section. Paleogeographic maps are useful tools for (1) demonstrating the distribution of depositional elements at certain times in the basin’s evolution; (2) checking for potential errors in the proposed correlation framework. Unnecessarily complicated maps of the depositional environments suggests that your method of stratigraphic correlation is incorrect. Work flow: (1) Place the center of each map box along the timeline #1 marked on each cross section. Mark the boundary for each depositional facies where it crosses the centerline of the map box. (2) Draw lines on each map defining the boundaries for each depositional environment. Paleogeographic Map for Time 1 based on Lithostratigraphic Correlation of deposits. Paleogeographic Map for Time 1 based on Parasequence Correlation of deposits. Questions: (1) Which paleogeographic map makes the most sense and why? (2) Does the parasequence set interpreted here possess a progradational or a retrogradational pattern?