Geological Map of Sithylemenkat Lake

Based on Airphotos

By Ray R. Collins

Is Sithylemenkat Lake an impact crater? That is the question I was examining when I made this geological map of the area. Originally it was identified by P. Jan Cannon as an impact crater based on aerial photographs. This map will examine his thesis and try to identify possible alternative explanations. The map is based on three aerial photographs, which allowed the lake to be centered in the geologic map. Some of the rock types are tentatively identified, based partly on the aerial photographs and partly on field work. Other aerial photographs were examined on either side of the study area to get the bigger picture, but were not included in the mapping.

Air photos of the area were obtained from the Geodata Center at the University of Alaska, Fairbanks (located in the IARC building). These were viewed as stereo pairs, and mapped based on surface features. Some data was collected on field trips to the area, which allowed more specific mapping of the units. Cross sections using USGS maps of the area were also done, to help identify possible structural controls.

1. Structure.

Based on the air photo I identified a fault that ran across the northern edge of the lake. Also identified were folded sedimentary or metamorphic rocks to the NW of the lake, which were not expected since USGS maps (Mull) identify the area as basaltic.

The stream is in a steep, fairly deep, antecedent valley (see photo). At first thought to indicate right lateral strike slip faulting, it was later determined that the stream is joint controlled based on field work which showed that the jointing paralleled the stream.

There is considerable jointing in the exposed plutons. Some of these are vertical, others dip in various (mostly northerly) directions.

Severe folding has occurred at some time in parts of the NW part of the map (colored brown). This is tentatively mapped as sedimentary, but may be metamorphic.

2. Units mapped

• Slanted yellow stripes. Shallow sloping alluvial terrain. These areas are of immature gravel and rocks eroding from the nearby mountain peaks.
• Dark green. Primarily low-lying taiga, covered predominantly with black spruce. Terrain is flat to gently sloping.
• Slanted purple lines. Basalt* which forms the barrier damming the lake.
• Short, heavy black lines. Solifluction lobes evident at the base of the mountain to the west of the lake.
• Light green. Wetlands located in the Kanuti River valley bottom (north and west of Sithylemenkat Lake), near Sithylemenkat Lake and at a alpine lake in the SW corner of the map.
• Yellow/orange. Sand beach around the perimeter of the lake. Identified (by field work) as being an immature sand, rich in potassium feldspar.
• Purple with black lines. Exposed bedrock. The lines indicate the strike of jointing. Vertical beds have a circle drawn through a line; dipping joint sets have short lines drawn in the direction of dip (the steeper the dip, the more the lines). Curved lines follow exposed tilted joint layers.
• The fault is identified with a dashed line, and runs across the NNW edge of the lake. It is believed that this is a part of the Kanuti Fault, or possibly a finger of it.
• Possible faults are identified with dotted lines, one separating the sedimentary rocks from the Kanuti river flats (note: this may not be a fault, but a contact, though it appears to cut across features rather than fill them in). The other one appears to angle NW off the west end of the "primary fault". This may actually be a junction between different slopes than a fault (ie the fault was buried by debris from the mountain).
• Mountain peaks are identified by +
• Hot springs, located in the center of the lake, were located from my own air photos (they can only be located in the winter because in the summer--when the aerial photographs were taken--there is no surface indication of them).
• Blue. Lakes and streams.
• Densely connected poly structures. Polygons in flat, boggy areas created by ice wedges.
• Black. Mountain tops, with no slope to medium (less than 15 degrees) slope.
• Sub-parallel lines, radiating from mountain peaks. Steeply sloping (greater than 30 degrees) gravel slopes.
• Brown. Sedimentary and/or metamorphic structures. Not positively identified; I'd like to do field work there to pin this down.
• Pair of lines, curved away from each other, labeled "pass". This is a low place between the mountains to the northeast and the rocky exposures to the west of the pass. This location was identified from the air photos as needing more field work to rule out the possibility of landslide or glacial moraine being the cause of the lake.

*initially identified as a pluton, but later identified by field work as being basaltic, at least in the outcrop nearest the lake. The outcrop to the west of the stream may be a different kind of rock (though it appears to be the same in the USGS aerial photographs).

Sithylemenkat Lake is located in a bowl shaped depression in the northern edge of the Ray Mountains in central Alaska, with the coordinates of 66° 08'N latitude and 151° 24W longitude. A bowl shape has been identified as one of the key features of an impact crater (Sharpton). The lake itself is very round. This could be caused by a structural phenomenon, or by a meteorite impact.

Seven possible causes of the Sithylemenkat Lake basin have been identified. Only one of these could be ruled out based on the aerial photographs, but the identification of a fault across the northern edge of the lake decreases the likelihood this feature is impact related. This fault, though it does not explain the round shape to the lake, could explain the lake (the round shape then being coincidence). The position of the stream, which is antecedent, argues against the fault being the cause, for if it were fault caused one would think the stream would have remained in the center of the valley. This is not the case, but could be explained by a left lateral strike slip fault moving the stream as the fault extended. The seven possibilities are, in order of likelihood: fault blockage of the valley; impact; regional tilting; glacial moraine; landslide; volcanic crater; cryptovolcanic feature.

Of these seven, regional tilting was ruled out by looking at aerial photographs of neighboring valleys. Though there is a lake about six miles to the west. However this is an odd shaped lake, most likely caused by landslide. Other valleys in the area do not show any signs of lakes or blockage that should be evident if the area were tilted enough to cause Sithylemenkat Lake.

The sand beaches were shoved up in an interesting fashion all the way around the lake. This is the work of ice, and it has created a very nice sandy beach. The sand has been shoved into marked piles (visible in the aerial photographs), which tend to dam water up behind them. Thus much of the area adjacent to the lake tends to be marshy.

The shore on the south side of the lake is not very round. But, upon inspection in the stereoscope, the aerial photographs show quite markedly that the lake used to be a great deal more round. The south shore of the lake has merely been covered with alluvium from the mountains to the south.

Impact structures tend to be very circular. This makes air photos an excellent way to identify structures that may be impact craters. Determining a structure actually is an impact crater has to be done with field work because the only way to positively identify an impact event is to find shocked minerals (usually quartz). Many of the impact structures that are known today were first located with air photos (Sharpton). Sithylemenkat Lake was first identified as a potential impact structure by P. Jan Cannon, based on air photos. I have attempted to expand on his hypothesis.

This exercise did not rule out impact as a cause of Sithylemenkat Lake. It did, however, identify a fault in just the right location to cause the lake. Therefore this fault becomes the most probably cause of the lake. Regional tilting was ruled out. Field work ruled out glacial moraines or landslide (bedrock was found outcropping near the "pass", which would have to be debris for either explanation to be valid). Cryptovolcanic features are unlikely in the extreme. The regional rocks (plutonic rocks to the south and surrounding the lake on three sides) tends to rule out the possibility of the structure being a volcanic crater.

So we are left with faulting and impact. Unless shocked quartz or some other feature identified with impact can be shown Sithylemenkat Lake may never be proven as an impact crater.

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