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.