Using facies models and sequence stratigraphy to model ...
Transcript of Using facies models and sequence stratigraphy to model ...
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William W. Little, Ph.D.W.W. Little Geological Consulting, LLC
Photo by W. W. Little
Using facies models and sequence stratigraphy to
model contaminant migratory pathways
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(Cramer et al. 2018)
Santa Clara (Silicon) Valley, CA
After 20+ years of remediation, contamination had increased at some monitoring localities down gradient from the source, while decreasing at others.
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Missouri River, MO
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Generalized model for Missouri River aquifer assuming homogeneous subsurface conditions.
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“All models are wrong; some are useful.”
Client: “What does your model say?”
Scientist/engineer: “What would you like it to say?”
Model Reliability
Models are interpretations based on preconceived assumptions (biases) into which data are entered.
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Standard Groundwater Flow Model
The subsurface is assumed to be homogeneous and contaminants are predicted to disperse as a simple plume starting at a point source.
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River ClassificationBraided Meandering
Anastomosed
Straight
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Facies Models for Fluvial Systems
Courtesy Samuel Wojcik
Courtesy Holly Mitchell
Meandering Fluvial Braided Fluvial
There are additional models for anastomosed fluvial systems, alluvial fans, and deltaic distributary systems, as well as much variation within these models.
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Flow Unit Connectivity
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Photo by W. W. Little
Photo by W. W. Little
Meandering Fluvial
Anastomosed Fluvial Braided Fluvial
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Variations on the Theme (Models)
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Added Complexity
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Variations on the Theme (Rock Bodies)
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Case Study 1: Santa Clara (Silicon) Valley, CA
decreasing at others.
(Cramer et al. 2018)
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Data
Facies Associations Contaminant Concentrations
(Cramer et al. 2018)
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(Cramer et al. 2018)
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(Cramer et al. 2018)
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Case Study 2: Chena River (Fairbanks), AK
Babcock, E. et al. (2015)
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Babcock, E. et al. (2015)
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What model does it fit?
Show anastomosed, meandering, braided, distributive
History
Incised, now filled
Facies Models
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Sequence Stratigraphy
In simplest terms, sequence stratigraphy is the depositional stacking pattern that results from one full cycle of base-level change. In landward areas, the lower and upper boundaries of a sequence are typically unconformities.
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Why is sequence stratigraphy important?
Correlate these four cores or measured sections.
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Lithostratigraphic Solution
Given the similarity in physical characteristics, the most common
solution would be to hang the sections at the top of the first prominent
sandstone.
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Sequence Stratigraphic Solution
An alternative would be to assume an overall progradational stacking pattern
common to highstand systems tracts and to hang the section on the first recognizable
time-correlative depositional event (parasequence), regardless of lithology.
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Incised Valley Fills
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Sediment bypassFormation of sequence boundary
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Photo by W. W. Little
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Case Study 3:“Pinnacle” Facies Associations
Before Sequence Stratigraphy
After Sequence Stratigraphy
Sadeque & Samuels (2018)