M. Helper
GEO420K
Assignment for data collected on 420K Field trip #5:
1)
Tabulate the measurements from all stop, with columns for
cleavage/foliation, joints, intersection lineation and mineral lineation. Clearly separate data from each stop. Turn is this table with the answers to the
questions below.
2)
Compare the mineral lineation data from Stop 2 to the intersection
lineation data from Stop 1.
Q1: Are the lineations
from the two localities parallel? If
not, how different are they?
Q2: If both
types of lineations are related to folding, on the
basis of your answer to question 1, what can you say about folding at the two
localities? Think critically about this
question; there may be more than one possibility. State your answer in terms of two hypotheses.
3)
Compare the same data to that at Stop 3.
Same questions: Are the lineations
from all three localities parallel?
If not, why not?
If so, what can you say about deformation at all three localities?
4)
Copy and turn in the sketches you have from Stops 2 and 3 that illustrates the relationship between folding and igneous
intrusions.
5)
Using the observations we talked about at all stop, your own sketches,
and the answers to the questions above make a chronological list of Precambrian
events. Where appropriate make reference
to your sketches and measurements.
6)
Turn in your pace and compass map from Stop 4.
This assignment is due
in class Wednesday, April 9.
M.
Helper
420k
Precambrian Rocks of the Llano Uplift:
420K Field
Trip #5 Stop Descriptions
Stop 1: Packsaddle
Schist metasediments near the base of the
Packsaddle Domain; road cut on NE side of
These thinly interlayed
gray, tan, black and light-brown graphitic and quartzose phyllites, biotite
schist and fine-grained quartzite are characteristic of the metasedimentary
package of rocks that marks the base of the Packsaddle Domain. Though the age of these sediments are
unknown, felsic metavolcanic
rocks elsewhere interlayered with the sequence
crystallized at about 1243 and 1248 Ma.
About 1 km north of this locality along Honey Creek, Packsaddle Domain metasediments are thrust over Valley Spring augen gneiss
that has an igneous crystallization age of 1270 Ma, beneath which there is orthogneiss containing zircons as old as 1360 Ma. Between this highway outcrop and the thrust
are interfolded layers of marble, calc silicate rock, graphitic phyllite, and amphibole schists. This same sequence and the graphitic
phyllites of this outcrop comprise the basement at
To be observed at this stop are:
1)
A well developed south-dipping cleavage that is axial planar to tight
to isoclinal folds of the compositional layers.
This cleavage is the dominant (S2) cleavage throughout the Packsaddle
Domain.
2)
Tight to isoclinal folds of the black, gray and tan phyllites,
demonstrating that although this "sequence" superficially looks
little deformed it is, in fact, highly repeated internally by folding. The folds plunge 28o toward S30E
and are thus best observed on surfaces at high angles to the outcrop. Much
larger scale examples of these same (F2) folds have been mapped by Reese (1995)
immediately north of here along Honey Creek.
3)
Lineations defined by the
intersection of compositional layers on the cleavage and by the alignment of
mica on bedding surfaces. Though the
cleavage and bedding are nearly parallel through most of the outcrop, the small
angle that exists between them results in mineral and intersection lineations on bedding and cleavage, respectively,
that are precisely parallel to the axes of the tight to isoclinal folds.
4)
Later generation, more open folds that fold the cleavage. These are similar in style and orientation to
the large, SE-plunging late-stage folds that produce the regional map pattern
of the Valley Spring and Packsaddle Domains.
5)
A small granitic pegmatite that intrudes
across and distorts the cleavage of the surrounding graphitic phyllite. Granite
plutonism here post-dates cleavage formation.
A thin bleached zone at the margins of the pegmatite indicates
volatilization of graphite during intrusion.
Exercise at Stop 1:
1) In 15 minutes,
standing on the southwest side of the Highway, sketch the outcrop and its major
features. Your sketch should resemble a cross
section, showing:
q the top and base of the
outcrop, with labels at the NW and SE ends and a rough scale bar
q the orientation of the
cleavage
q any major lithologic differences within the outcrop
q any faults or folds
q the pegmatite
2)
In 20 minutes, with your Brunton, measure:
q the orientation (strike
and dip) of 4 cleavage surfaces
q the orientation (trend
and plunge) of 4 intersection lineations
q any fold hinge lines
that you can find
q the orientation of 5 joint planes
Stop 2: Valley Spring Gneiss, at
north bridge abutment of the Highway 16 bridge across the
Gray- and pink-banded gneiss here show superbly
developed mineral elongation lineations on foliation
surfaces that define broad, open folds.
The gneiss is intruded by two generations of granite that either
cross-cut or are folded with the foliation.
This outcrop permits definitive observations of the relative sequence of
events (plutonism, deformation, metamorphism) that
define the Precambrian history of uplift.
To be observed here:
1)
Gneiss, a gneissosity and a schistosity
2)
The mineralogy of the gneiss
3)
Granite dikes and a sill
4)
Folds that affect granite and gneiss
5)
Boudinage of a granite sill
6)
Joints related to folding
Exercise at Stop 2:
1) In 15 minutes, standing to the north of the outcrop, sketch a E-W cross-section of the outcrop showing its major
features. Your cross section should
show:
q labeled W and E ends
and a rough scale bar
q the orientation of the foliation throughout
the outcrop
q granite dikes and
sill(s)
q folds, paying
particular attention and showing what rock types are folded
2)
In 20 minutes, with your Brunton, measure:
q the orientation (strike
and dip) of 4 foliation (schistosity or gneissosity)
surfaces
q the orientation (trend
and plunge) of 4 mineral elongation lineations
q 2 fold hinge lines
q the orientation of 5
joint planes
Stop 3: Valley Spring Gneiss and granite, Devil’s Water Hole at
Spring Creek,
The gneisses at this stop display evidence of migmatization and polyphase
(multiple events of) folding that are bracketed in time by the intrusion of
pre- and post-deformational granite sills and dikes. The post-folding sills and dikes are
isotropic aplites, pegmatites
and coarse-grained pink granites that are believed to be related to the
The earliest folds to see here are isoclinal folds
of the foliation. These are overprinted
by at least one later phase of ductile folds that are asymmetric and tight to
isoclinal. Mineral elongation lineations are associated with at least one phase of these
earliest folds.
The youngest phase of folding is recorded by more
open, upright, southeast plunging folds.
These folds fold lineations are perhaps the
most common, easily recognized structures in the area.
To be observed here (time permitting):
1) Two varieties of gneiss
2) Foliated and unfoliated
granite dikes and sills
3) Migmatite
4) Strongly lineated gneiss
4) Larger-scale folding
5) Field observations recorded on a map of the
area
Exercise at Stop 3:
1)
In 20 minutes, with your Brunton, measure:
q the orientation (trend
and plunge) of 4 mineral elongation lineations
q 2 fold hinge lines
2)
In 30 minutes, make two small sketches of field relationships that demonstrate
the timing between deformation and plutonism at this locality.
Stop 4:
Exposed
here is coarse-grained granite of the Kingsland Pluton, the central body of a
group of three very similar and nearly coextensive plutons (others are Lone
Grove and
To be observed at this stop are:
1)
The texture and mineralogy of the granite
2)
Granite dikes and textural variations within the granite
3)
Joints
Exercise at Stop 4:
1)
Time and exposure permitting, we will construct a pace and compass map
of the granite pavement. Features to map
are:
q The edges of the
outcrop
q Any dikes, textural
changes or other features within the granite
q Joint sets
2)
Measure the orientation of
q any granite dikes
q Joint sets