AboutGeorge Ardies Expertise Sedimentology and Petroleum Geology - I am not an expert in igneous and metamorphic geology. I have completed a B.Sc. in Environmental Geology at Concordia University in Montreal, Quebec and a M.Sc. degree in Sedimentology at Queen`s University in Kingston, Ontario. I have about 12 years of experience in oil and gas exploration and development.
Experience Military service (Montreal, Quebec, Canada).
Bachelor of Science degree(Montreal, Quebec, Canada).
Master of Science degree (Kingston, Ontario, Canada).
Oil and gas experience for about 10 years (Calgary, Alberta, Canada).
Professional Geologist Certification.
Question Hello George,
In the Rocky mountains, near the continental divide, there are numerous valleys with very odd deposits. The mountains give way to gently sloping terraces that extend from the foot of the mountains to sometimes near the center of the valleys. The toes of these deposits are nearly always quite steep, perhaps 100 to 50 % slopes, with very gently declining tops that are almost but not quite level. The terraces are sometimes stepped, and sometimes not. The larger terraces stand easily 2-4 hundred feet above the valley floors, and if you saw them from the air, they resemble broad aprons, almost like alluvial fans, but not originating from a point source like so many fans do. The tops of these terraces are now days host to hay fields, and cow pastures.
Now here's the kicker. I have examined quite a number of these things, and although the material may be different, depending of the mountain range of origin, the deposition sequence is often times very similar. Here, I offer a typical description of one of the larger terraces, since they are often incised by stream courses, and sometimes even whole sale mass wasting if too much water is applied to the hay crops atop these darned things.
The top soil is quite thin, perhaps 4 to 10 inches on average. Beneath that, the sub soil of similar depth, is a leached layer of fine clay like material, though it is not competent enough to stick together in a ball when wet. Next comes big gravels composed of water worked stones with an average diameter of 5-7 inches. I have examined this layer in different locations and it can be anywhere from 4 to 12 feet deep. Although the boundarys are sometimes sharp, the bottom of these "gravel" beds often grade to finner gravels with poor bedding inter spaced with lenses of silts, sands, and clays. This layer can be at least as deep as the more homogeneous cap of large "gravel". All this stuff sits atop massive fine clays, sometimes hundreds of feet thick, with some occasional bedding, but not always. Of course, I don't know what's below that, since erosion is actually filling rather than exhuming these strange, arid, high altitude valleys. These terrace deposits are, in a some few places, over laid by recent glacial tills, at least in the highest, northern most valleys. I have thought these deposits to be a bit of a mystery ever since I first laid eyes on them years ago. What's your take on all this?
Answer Thanks for the great description Morgan,
Your knowledge of geology is extremeyl good. In my opinion, the underlying clays may represent low energy deposition, possibly that of an estuary. The gravels may represent some sort of flood event leading to very high energy deposition of the gravels and poorly sorted deposits. The fact that there does not appear to be a point source suggests fluvial deposition. Now, having said that, if the deposits are located near a marine body/shoreline/strand plain, then the cobbles may represent a high energy beach??
There are a lot of these types of deposits along the fjords of Norway and northern Canada. The do genereally represent high energy flood deposition. The glacio-isostatic rebound then "lifts" the area, which is why the terraces stand so high above the modern water level.
This is just my "take" on things. Having not seen the deposits, it is an educated opinion. PLease let me know if this description fits your observations of the rocks.
