GLG 240, Dr. Riggs
LITHOLOGIC DESCRIPTIONS
Describing rocks is one of
the most important things you will do in geology, regardless of whether you are
an environmental scientist or a volcanologist. It takes time and practice, but you have to learn to do it
right.
Today we will look at an
igneous rock and two kinds of sedimentary rocks (clastic and chemical). Next week we will finish looking at the
stratigraphy of the Flagstaff area.
You will fill in the description ÒtemplatesÓ provided here for the rocks
we see today, and you must write a description of the units we will look at
next week AS WELL AS a rock from
next to BLDG 13, in your own words.
Stop 1: Moenkopi Sandstone (Early Triassic)
(Station number )
The
Moenkopi Sandstone is exposed ( /4,
/4, sec. , T N, R W).
The Moenkopi Sandstone is bedded (can you see beds clearly, are
they distinct (Òwell beddedÓ), irregular, do they pinch out)?. The outcrop is m thick. Overall, the unit forms (slopes, ledges, combination). [n.b., these are things you can observe without going
anywhere near the outcrop)
The
Moenkopi is (color) on weathered surfaces and on fresh surfaces. Ledge-forming beds are sorted (well, poorly, moderately) grained (coarse, fine, very fine?) (sandstone/mudstone/limestone/ash-flow
tuff) and consist of (what minerals can you identify?) in a cement (silica? carbonate?). These beds are (hereÕs where you fill in some
information about sedimentary structures -- cross bedding, ripple marks, graded
bedding (see the other sheet)).
Slope-forming beds consist of (sorting, size) grains of (minerals) in a matrix. (fill
in the things about sedimentary structures and bedding). The (aspects of sedimentary structures,
color, or whatever) indicate that the Moenkopi Sandstone was deposited in a environment.
The
strike and dip of the Moenkopi Sandstone at this locality are ,
and the sketch shows (a sketch should be drawn to illustrate
some point you want to make -- that beds do pinch and swell, that ledge-forming
beds and slope-forming beds are intermixed, that cross-bedding is very common
and high angle, that welded pumices are common throughout the unit).
Stop 2:
Moenkopi Sandstone and ?. (station )
(n.b., when you are writing
your report, does this description come before or after the previous one?)
A
contact between Moenkopi Sandstone and (fill
in some general descriptor. If you
know what this is automatically, great, but if you donÕt, all the better!) is
exposed (complete
location information, and most ideally with some reference to any previously
described outcrop that is relevant (youÕre looking at Moenkopi again, so itÕs
useful to describe where you are with respect to the other place you looked at
Moenkopi). The Moenkopi Sandstone
is bedded, and the outcrop is m thick. Overall, the unit forms . The overlying unit is m thick and is a former
(see the Moenkopi Ss description for what to fill in where. In general, at the risk of being really
boring, itÕs useful to describe things in the same order from formation to
formation).
(these lines are for you to write a
description of the Moenkopi in this area, using your previous description as a
guide).
The
contact between the Moenkopi Sandstone and the overlying unit is cm thick. Compared to the Moenkopi, the contact zone is (easily eroded, resistant to
weathering). The material along
the contact consists of (you
know what to do! grain size,
mineralogy if you can, sorting, sedimentary structures, etc.).
The
uppermost unit in the area is (it may take a description to be
able to fill this in, but in general, itÕs nice to start a description with
what the unit is -- a name for it).
The
is on weathered surfaces and on fresh surfaces, and is approximately m thick. It consists of approximately % crystals
that are (size), and % crystals
that are (size). The groundmass is (you may not have a good idea how to
describe the groundmass -- please ask!!)
Vesicles are (common/rare)
at the base of the flow and (decrease/increase) in abundance upward.
Basalt
in the Flagstaff area is approximately 6 million years old. Thus, the contact between the Moenkopi
and the basalt represents a (what kind of unconformity?). (free
space to use your imagination about the contact and what happened during the
time it formed -- what the material actually is!). [If you chose to draw one of your sketches here, be sure to
write some words about it.]
Stop 3:
Kaibab Limestone (Permian)
(the regular stuff describing where the
Kaibab Limestone is exposed). (more
regular stuff to describe the bedding, the thickness of the outcrop, and the
slope/ledge-forming nature of the formation). (words
on color), and consists of (size)
crystals of (calcite,
dolomite?). Sand grains are %
throughout the unit. Sedimentary
structures include (if
there are any), and fossils of are present.
The
strike and dip of the Kaibab Limestone is . Based on ,
the Kaibab Limestone is interpreted to have been deposited in a(n) environment.