| Click here for some photos of the field trip to Horne Lake |
I have excerpted the following
statements from a description of the Upper Pennsylvanian Mt. Mark limestone at
Horne Lake by Ken Porteous.[1]
Read the statements, and then answer the questions below. Please get in
touch with me if you need help.
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Biosparite
(p.
10 and 11) |
|
|
a) |
“The dominant facies is … comprised of 50% skeletal allochems, 44% spar, 5% quartz, 1% organic material …” “The skeletal allochems of various sizes are floating without contact, suggesting that the spar is a secondary diagenetic replacement of a finer material, probably an ooze matrix.” |
|
b) |
“The
skeletal allochems appear to be abraded only slightly as evidenced by some
rounding of corners ….” |
|
c) |
“…
the lack of any ooids rules out wave action in the intertidal zone.
This compares favourably with a subtitdal lagoon or shelf
environment where storm wave action may occur occasionally.”
|
|
d) |
“The
lack of any biolithites also tends to support a shelf or lagoon
environment lateral to any reefs.” |
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|
|
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Biomicrite
(p.
11 and 12) |
|
|
e) |
“The
allochems are comprised of ~75% skeletal allochems that show some
rounding of corners …” “This indicates a packed biomicrite … whose
grains were transported from source to a depositional basin…” |
|
f) |
“The
depositional basin must have been free of all but the weakest of currents
for the microcrystalline ooze to remain in place and for there to be no
signs of ooids present.” |
1) Explain the underlined phrase in
(c), and explain why this statement and statement (f) are reasonable.
2) Rephrase statement (d) using words
that a non-geologist could understand.
3) What is the significance of the
rounding of skeletal allochems described in statements (b & e)?
4) Explain how the underlined phrase
in (a) supports the contention that the spar is of secondary origin.
(“Floating without contact” is the same as saying that the allochems
are not touching each other.)
[1] Porteous, K., 2003, An Upper Pennsylvanian fauna at Horne Lake on
Vancouver Island, Malaspina University-College, GEOL-490 paper,
55 p. (This paper is available
in the classroom.)