Table 1. Observations and Characteristics of Sedimentary Layers This photograph
ID: 286892 • Letter: T
Question
Table 1. Observations and Characteristics of Sedimentary Layers This photograph shows a sequence of layers. Try to identify boundaries between different sedimentary rock units, and draw a dashed line on the photograph along any main boundary. The stratigraphic section to the right contains some descriptions of the units shown in the photograph and refer to additional photographs to the right of the stratigraphic section in the textbook. You do not have to write any observations on this worksheet, but your instructor may have you discuss these in class Stratigraphic Section This unit includes sandstone, mudstone, and layers of coal (shown in black). The upper part of the unit contains sandstone beds with small cross beds. The mudstone has mudcracks and plant fossils. The lowest part of the unit contains tan sandstone with broken marine shells. This photograph shows thin layers of black coal in this unit. This shale is medium to dark gray because it has a high amount of organic matter. It contains fossils of clams and other marine organisms. Thin limestone beds are locally present in the middle of the unit but are not shown in the section. The shale and limestone contain abundant marine fossils. The photograph shows a close-up of the transition from the shale to the overlying sandstone. This unit is mostly a yellowish-tan sandstone containing quartz sand with small pieces of marine shells. As shown in the photograph the very base of the unit is a thin conglomerate that overlies a sooured erosion surface. This lower part locally contains fossils of wood and leaves. The lowest unit includes a conglomerate with moderately rounded peb- bles and coarse sand containing scattered rounded pebbles and pieces of fossilized wood. The conglomerate is overlain by reddish, maroon, and gray shale and mudstone with plant fossils. This photograph) shows a nearly circular dinosaur track where a large, plant-eating dino- saur with huge round feet stepped into and pressed down the then-soft sediment. 071661Explanation / Answer
Ans 1: The change of environment from the base of the section (coarse-grained sediments) up to the thick gray shale (finer grained rocks) indicates transgression of the sea or landward movement of the shoreline. This is because transgression would lead to deposition of finer sediments (from offshore areas) over the underlying coarse-grained rocks of on-shore areas. This would lead to the formation of a fining- upward sequence (basal conglomerate--> sandstone --> shale).
Ans 2: The change from the thick gray shale (fine-grained) to the overlying sandstone (coarser grained) indicates regression of the sea or seaward movement of the shoreline. This is because regression would lead to deposition of coarse-grained rocks (from on-shore areas) over the underlying finer grained rocks in the offshore areas. This would lead to the formation of a coarsening- upward sequence (shale--> sandstone).
Ans 3: Stratigraphically, transgression occurs first and then regression. Thus the correct answer will be: (c) transgression followed by regression
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