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The relationships 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 complied vertically, they represent a sequence of environments produced by a specific over-all sedimentary process such as regression or transgression. These environmental successions are definite and recognizable and may be used to determine fundamental depositional sequences in ancient rocks. Therefore, depositional sequences may be used as reference frameworks (or models) for interpreting the sedimentary history of any stratigraphic section.
Only a limited number of depositional sequences (or models) has been found in a comparative study of Recent and ancient sediments. The following sequences have been developed into models of sedimentation.
4. Marine shale or lagoonal deposits
3. Thick-bedded, wavy-bedded, well sorted sand
2. Thin-bedded sand/silt and shale
1. Poorly bedded shale
Lowermost units are deposited below wave base; higher units close to sea-level.
Fluvial (channel or valley-fill)
4. Ripple cross-laminated zone
3. Laminated, even-bedded sand/silt zone
2. Festoon cross-bedded zone
1. Basal conglomeratic zone
A regressive marine sequence overlain by nearshore fluvial, bay, and marsh deposits; characterized by extensive marine deposits and variable non-marine sediment distribution patterns.
2. Marine shales or fragmental limestones
1. Sand or silt
Widespread distribution and thinness are diagnostic; basal unit abruptly overlies shale in many places; gradation upward into marine shale is rapid.
Widely distributed, thin-bedded; fauna and sedimentary structures characteristic of deeper water.
Resemble regressive marine sequences, but are commonly developed on a smaller scale and lack marine fauna.
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