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The relation of the vertical stratigraphic succession to areal depositional patterns was first observed by Johannes Walther and is known as Walther's Law of Facies. Each depositional unit in a vertical stratigraphic succession is the result of a particular sedimentary environment. When these units are compiled vertically, they represent a sequence of environments characteristic of a specific over-all sedimentary process such as regression or transgression. These environmental successions are definite and recognizable and may be used to define over-all sedimentary processes in ancient rocks. Therefore, environmental sequences provide a reference framework for interpreting the sedimentary history of any stratigraphic section.
Only a limited number of specific vertical sequences has been found in a comparative study of Recent and ancient sediments. The following sequences have been developed into models of sedimentation.
c. Marine shale or lagoonal deposits
b. Thin-bedded sand/silt and shale
a. Poorly bedded shale
Lowermost units are deposited below wave base; higher units close to sea-level.
Fluvial (channel or valley-fill sequence)
d. Ripple cross-laminated zone
c. Laminated, even-bedded sand/silt zone
b. Festoon cross-bedded zone
a. Basal conglomeratic bedload zone
Regressive marine sequences overlain by nearshore fluvial, bay, and marsh deposits; characterized by extensive marine deposits and variable non-marine sediment distribution patterns.
b. Marine shales or fragmental limestones
a. Sand or silt
Widespread distribution and thinness are diagnostic; basal unit abruptly overlies shale in many instances; gradation upward into marine shale is rapid.
Widely distributed, thin-bedded; fauna and sedimentary structures characteristic of deeper water.
Resembles regressive marine sequences, but is developed on a smaller scale and lacks marine fauna.
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