Field Methods week 5 –
 Bayonne Pk – more?
 High Point – lodging?
 Newfoundland coming up – draw a scale cross-sec
I. Crash course in struc geology
A. bedding attitudes – all kinds
tell stories of deposition, deformation
B. collect the DATA first, then interpret
recognize you always have multiple wkg hypotheses in your head as
you map
C. sed strucs critical for history, envir of deposition, placement in regional
depos settings (eg, Walther’s Law of Facies migration) AND
Which Way is UP? What is OLDER? YOUNGER?
CAN’T determine a bedding attitude without knowing which way is up
cross beds - which way is up? no problem
unconformities – which way is up? maybe a problem
Mudcracks and subsequent fill – no problem if you know which is first…..
wk 5 p.2
so UP is critical, because it controls your symbology (S&D) and the
symbology controls your interpretation
D. related to UP and sed strucs…bedding planes
these are what you want to measure FIRST, then worry about
fracture systems
BIG PROBLEM IN THE FIELD – differentiate bedding from fractures
talk to Mira & Alex about Fairview Lake…….
yet another reason why the bedding attitude ID is SOOOOOO important
E. Struc history
start with
end up with
so now there’s a whole new set of problems – faults and folds..how
represent?
understand the bedding and map it out…that is the key
Wk 5 p.3
II. measuring attitudes with the Brunton
Strike – the declination of a horizontal line on the plane you are measuring
(a bed, a fracture, a fault….)
Dip – the maximum angle of slope in degrees down from horizontal, often
with the direction (declination) toward which it is dipping
North
25
strike = N 45 E
dip = 25 SE
on a monocline, this is no problem – all beds tend to strike and dip in same
direction
with folds, things get more complex…….
imagine a cylinder, horiz axis
strike for both limbs is…?
and dip might be……
Now take cylinder, and plunge the nose into the ground, then imagine how
that structure intersects the Earth
here you see the bedding plane S&D describe an arc around the center of
the fold
when in the field, you are measuring these attitudes, trying to construct in
3D what these things look like….and how they got that way……
At Newfoundland, we have a great exposure, anticline-syncline pair, and a
fault or two……this is laid out for you in x-sec, but tough to walk it out in
map view
At High Point, the exact opposite is true….you’ll map out the bedding
attitudes by finding the outcrops, try to figure out what’s up, down, older,
younger. the YOU will develop the cross section(s) that will show the true
3D geometry.
so let’s get some practice in, start learning some rules…..
wk 5, p.4
III. Symbols – what do I use???
2 separate but related issues involved with what is older & younger vs up
and down
need to understand anticlines and synclines…..oldest in center vs youngest
in center
normal sequence, no problem
Dev
Sil
Ord
- youngest
up
C
PC – oldest
what’s are these?
Dev
Sil
Ord
- youngest
up
C
PC – oldest
now it gets worse…overturn the beds now, and oldest may be uppermost –
have to have symbols that work for that too