The Foscoe Lacustrine Facies

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The Foscoe Lacustrine Facies?

• Anthony Love
• Department of Geology
• Appalachian State University
• Boone, NC

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Purpose of study

• To examine the some of the properties that
• the Foscoe Lacustrine facies (FLF) were
• identified by
• laminations and layering (bedding)
• presence of carbonate minerals deposited
• in laminations

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Purpose of study
2. To experiment with and use SEM to aid in
understanding of these rocks and help guide
future research. experimentation with scanning
electron microscope attempts at analysis
using -secondary electron imaging
-backscatter electron imaging -characteristic
x-rays -cathodoluminescence.
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Methods of study field mapping and sample
collection petrographic examination of
structures and minerals -with light and electron
microscopy comparison with models of
lacustrine facies rocks discussion of effects
of metamorphism -with relation to rock
structure, textures of minerals and their
distribution
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Format
I. general geology of Grandfather Mountain II.
discussion of Lacustrine Facies rocks III. a
synopsis of observations provided by field
mapping and light microscopy IV. Some questions
and ideas for application of EM V. Discussion of
direction for further study VI. Conclusions
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Definitions primary- structures and
textures that form when the original rock forms
layering- a descriptive term referring to
dark and light bands laminations- thin
alternating primary layers
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Grandfather Mountain Geology
Geologic Map of the Grandfather Mountain
Window (highlighted in green) and surrounding
rocks (Stewart, 2003)
Neoproterozoic (Precambrian) metamorphosed
volcanic and sedimentary rocks deposited as a
rift basin sequence (Schwab, 1977, 1986a Boyer,
1978).
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Grandfather Mountain Geology Grandfather
Mountain Window
rocks are exposed in the Grandfather Mountain
Window (highlighted in Green) within the Blue
Ridge thrust sheet. The window is a fault-bounded
erosion structure (Bryant and Reed, 1970a,
1970b)
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Grandfather Mountain Geology
rocks are metamorphosed to greenschist facies
and show evidence of multiple periods of
deformation and recrystallization (Bryant and
Reed, 1970b, Odom and Fullagar, 1984, Raymond and
Love, 2005)
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Lacustrine facies sediments
Idealized stratigraphic section for a short lived
lake
Lacustrine deposits form in landlocked basins
usually with freshwater input. a typical rift
basin sequence of coarsening upward sediments
(as the basin fills) consists of
-alternating thin layers of coarse and fine
sediment (sometimes with carbonate sediments
present) -sands -gravels
conglomerates sands sands sands limestone sands
limestone peat sand alternating layers of
limestone and peat representing seasonal
cyclicity and water shifting
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Location and extent of Foscoe Lacustrine Facies
Study area
Southeastern United States
Stewart, 2003
www.terraserver.microscoft.com
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Foscoe Lacustrine Facies
Interpretation of the Foscoe rocks has been based
on sedimentology silt, sand, and carbonate
layers (which were later metamorphosed)
presence of calcite and the possible presence of
dolomite. observed laminations recognized
and named on basis of mineralogy, structure,
distribution and context within regional
geologic setting (Bryant and Reed, 1960, Brown
et.al, 1985, Hageman and Raymond, 2000 personal
communication)
A. Love Photo
photomicrograph showing laminations, foliation
and kink bands (100X in plain light).
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• Questions
• Are there primary structures (layering and
  laminations)
• in these rocks that might indicate their
  origins?
• Is the source of carbonate in these rocks
  primary?
• 3. What types of carbonate minerals are present?

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Layering Primary sedimentary rocks have been
metamorphosed and recrystallized into
metasiltstones with lenses of metasandstone and
carbonate minerals. rocks have foliation
which is enhanced by alternating darker and
lighter layers
A.Love Photo Dr. Steve Hageman examines a
block of the FLF
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Layering Darker layers consist of opaque
minerals intergrown with deformed phyllosilicate
minerals. lighter layers consist of deformed
quartz and alkali feldspar crystals. Some
lighter layers contain concentrations carbonate
grains.
muds
carbonates
sands
A.Love Photo
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Deformation
As strain is partitioned in rocks, zones of
weaker minerals accommodate strain more easily
than stronger minerals (Raymond and Love, 2005)
A.Love Photo crenulation folds in white mica
and biotite
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Layering and Lamination
layering may be enhanced as weaker minerals
recrystallize and flow around layers of stronger
minerals layering may be accentuated with
continuing deformation enhanced layers have
the appearance of laminations
A.Love Photo
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Mineralogy and Metamorphism
CO
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Greenschist facies metamorphism of pelitic rocks
with carbonate layers yields a stable mineral
assemblage of Quartz (Qtz) Alkali Feldspar
(AFS) White Mica (WM) Biotite (Bio)
Calcite (Cc) Ilmenite and/or Magnetite
Titanite Epidote Chlorite Foscoe rocks
contain Qtz, AFS, and Cc with minor amounts of
Epidote and Sphene(Titanite) in the sandy
layers and Bio, WM and Ilmenite and/or Magnetite
in phyllosilicate layers.
Qtz
WM
Bio
Bio
A.Love Photo
WM
CO
3
Qtz
Bio
A.Love Photo
CO
3
Qtz
WM
A.Love Photo
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The Foscoe Lacustrine Facies Mineral Growth
Textures
Neocrystallization and multiple episodes of
recrystallization and deformation can be observed
in quartz, calcite, white mica, biotite, ilmenite
and epidote
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The Foscoe Lacustrine Facies Mineral Growth
Textures
Calcite(Cc) grains and veins show the greatest
diversity of textures (deformed and
recrystallized)
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3
3
A folded grain of calcite
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The Foscoe Lacustrine Facies Mineral Growth
Textures
A recrystallized grain of calcite
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The Foscoe Lacustrine Facies Mineral Growth
Textures
A stretched grain of calcite
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The Foscoe Lacustrine Facies Mineral Growth
Textures
A recrystallized equilibrated grain of calcite
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Observations from Light Microscopy primary
layering and laminations have been distorted and
enhanced by deformation and recrystallization
new metamorphic layering is enhanced by
accomodation of strain and progressive
deformation during metamorphism as
deformation continues, the resultant layering
becomes more pronounced enhanced layers
(structural laminations) have been misinterpreted
by geologists as primary layering
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Observations from Light Microscopy Cont. both
recrystallization and neocrystallization
indicate -re-equilibration under new
conditions and -either mobilization of some
mineral phases (carbonates)/ or introduction
of new mineral phases by fluids / or both
processes, during metamorphism
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Scanning Electron Microscopy of Foscoe Rocks
Question What techniques could provide valuable
information about these rocks and their
history? Experimentation was undertaken using
the following procedures Secondary Electron
Imaging Backscatter Electron Imaging Energy
Dispersive X-ray analysis Cathodoluminescence
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Secondary Electron Imaging
Highlights of SEI provide some information on
distribution of opaque minerals (brightly
relective grains dispersed throughout
sample). gives sense of layering, foliation
and folding. Problems and Concerns
comparison of the above SEI with a
photomicrograph of the same area offers no
clues about distribution of charge in brightly
reflective areas of the SEI and how it might
relate to composition.
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Backscattered Electron Imaging
Highlights of BEI can image compositional layeri
ng. Problems and Concerns surface imaging
of polished sections makes mineral
identification difficult due to lack of relief
contrast
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Energy Dispersive X-ray analysis
Highlights of EDAX allows excellent chemical
analyses without destruction of sample.
identified some carbonates as Calcite. holds
potential for answering questions of mobility of
certain mineral species Problems and
Concerns data must be interpreted in context
of standards (which were absent in this
experiment) accurate targeting of grains can
be tricky and time consuming.
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Cathodoluminescence Imaging
Highlights of CI allows visibility of minerals
not appearent with other SEIs or BSEIs
provides information on mineral growth (zoning or
recrystallization) especially in carbonate
minerals allows more accurate targeting of
mineral species to analyze with the Energy
dispersive X-Ray
A.Love Photo
Problems and Concerns other methods of sample
preparation need to be explored to gain
information about crystallization. Further
experimentation is required for coated vs.
uncoated specimens
A.Love Photo
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Discussion Additional techniques that may
further this study -more sampling in coarse
sand layers -staining of specimens to discern
varieties of carbonate minerals -more thorough
analysis of mineral chemistry using
standards might give more concise information as
to the role of fluids vs. mobilization of
elements -charge contrast imaging may also
provide additional information to that provided
by CLI about the nature of crystallization.
Unanswered Questions -if carbonate minerals are
not primary, what are other sources for these
minerals (faults, other rocks, groundwater)??
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Conclusions The interpretation that these
Foscoe rocks do represent a lacustrine
facies fits in with the regional geologic setting
of the Grandfather Mountain Formation, although
their history remains problematic due to the
effects of regional metamorphism and
deformation. The layering and laminations
result from metamorphic events rather than
primary deposition. Field and microscopic
observations indicate that these rocks have
experienced many periods of recrystallization
and subsequent requilibration of mineral species.
These effects result in a wide range of textures
of minerals.
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Conclusions cont. The range in crystal growth
textures of carbonate minerals indicates that
both mobilization during metamorphism and
introduction through fluids are likely to be
important processes in the occurrence of this
mineral in these rocks. SEM can delineate
some textural features such as layering with SEI
and BSEI Cathodoluminescence can be useful in
mapping flat rock chips for further analysis
with EDAX as well as for determining mineral
zonation. Calcite was identified as a
carbonate phase but further work needs to be
undertaken with the use of mineral standards to
answer questions about primary or secondary
carbonate minerals and their sources.
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Thanks are in order to Dr. Ruth Dewel Dr.
Loren Raymond Dr. Steve Hageman Appalachian
State University