The Bonneville Flood and the Wasatch Fault

this is the gk gilbert geologic park in utah the wasatch falls is about 200 meters behind me here my name is rick speden i'm here to share with you a geological murder mystery it has a lot of intertwined subplots and they all build towards a singular conclusion the wasatch fault runs from idaho down through central utah pleistocene lake bonneville occupied part of the basin and range region to the west the salt lake segment is the basin between the okra mountains and the wasatch we are interested in the section between big cottonwood and little willow canyons technical publications and educational materials consistently refer to the dramatic example in this area of graben or dropped fault block the grabbin theory first started with gk gilbert 130 years ago i will refer to this feature as the gilbert trough rather than a craven this is looking north into the gilbert trough across the mouth of little cottonwood canyon notice the towering and well-defined main scarp on the right glacial tail in this area is a very stable material wasatch boulevard runs through the depression the homes on the left are at the top of the anaesthetic scarp some of the research by others sets the stage for this story hardy and low determined that in the presence of high water table the thick layer of deposited sediment which formed as lake bonneville rose is subject to liquefaction during an earthquake in 1981 swann did trench studies on the main and antithetic scarps in the gilbert trough he found that the main scarp was formed over a number of earthquakes but the antithetic or gravin scarp was formed in a single event labs dated the most recent glacial maximum which crossed the wasatch fault his results put that maximum to within a thousand years before the bonneville flood based on others dating of the flood event mcalpin oversight deep trench excavation of the main scarp of the gilbert troth in 1999 he determined that there was an initial earthquake event t which occurred when the fault was submerged by lake bonneville and that event t was the only major earthquake on this segment which occurred when lake bonneville was anywhere near its high standard allowing for time for the glaciers to recede off of their maximum event t would have occurred within the last few hundred years leading up to or potentially coinciding with the bonneville flood the timeline shown here starts about 35 000 years ago and shows the gradual formation the liquefactions susceptible transgression like bed the climate then turned colder glaciers returned to the wasatch and deposited what the utah geological survey described as deltaic sand and silt locally weakly cemented with calcium carbonate sometime after lake bonneville achieved a high stand the glacier started to recede the lake level in this period is a matter of debate the evidence presented here suggests that the lake remained at or near its high stand right up to the bonneville flood physics dictates that event tea would have initiated basin surging in lake bonneville and possibly also a station a tsunami the following evidence supports at a minimum full depth west to east basin surging the wasatch front in the area of interest forms a natural cove would tend to focus the energy of a west to east surge and then deflect it to the northwest across slope creating an eddy in the dry creek cove if we focus on a pre-development photo of this area we find a geological anomaly what i am calling the dry creek island it is an island of glacial till material displaced from the mouth of south fork dry creek canyon to the northwest by half a kilometer this island is surrounded by lake transgression sediment known for liquefaction when submerged if this island had simply slid into a new location by gravity it would have done so downhill to the west instead it moved cross slope in concert with the predicted reflected flow lines of massive lake surging during a liquefaction-inducing earthquake the same type of shifting of glacial till occurred in the gilbert trough using google earth we can create an elevation profile and we can project the slope of the transgression lake bed up to where it intersects the fault line the trigger point of the shift was the intersection of the fault line and the weak strata the main scarp then comprises two components a slide component and an earthquake scarp component the earthquake scarp component is 15 meters high which matches the earthquake scarps in other parts of the salt lake valley this analysis also suggests that there is five to seven meters of holocene fill which has accumulated in the bottom of the gilbert trough over the last eighteen thousand years in dr mcalpin's 1999 trench work he did a core sample at the base of the gilbert trough and found just that five meters of holocene fill over lake bed sediment note if this was a graven it should have shown holocene fill over vertically displaced glacial till i've run similar profiles as sites to the north with similar results the glacial marines of little cottonwood and bells canyon have been used as classic examples of a graven displacement however a review of those marines in light of this new theory explains previously overlooked features these marines also slid out on the transgression lake bed sediment with the trigger point being the fall lake bed intersection this can be seen very clearly on the southern belles canyon moraine scarp the bell's moraine split in two with the northern section sliding in unison with the little cottonwood moraine if we walk back the slide and earthquake scarf displacements in this google earth profile around the bells canyon marine high points we end up with a very well-shaped moraine the directions and approximate displacements can be calculated the split created the very distinctive comma-shaped fissure on the face of the bell's terminal moraine the reason for this strange comma shape goes back to the bull lake glacial episode of 130 thousand years ago in that episode the moraines extended farther into the valley and vestiges are still visible today the fish are shifted to follow the bull lake northern lateral moraine this scenario also protects another fissure in the little cottonwood delta and indeed there is a depression along that line today the little cottonwood moraine slid out in concert with the northern section of the bells canyon marine matching slide scarp and anaesthetic scarf lines can be found on each the earthquake scarp is a very distinctive easternmost scarp on each of these marines the rest are just slide scarps the little cottonwood canyon marine can be reconstructed using data from a google earth profile backing off the displacements we get the original shape and then with a simple two-dimensional mass balance we see that the missing material is accounted for and filling the void created by the fissure down to the displacement plane of the lake bed three other geological features are worth mentioning first the case of the missing delta the mouth of little cottonwood canyon should have had a full width glacial till delta instead the center of it is missing this was not due to stream erosion because the very stable glacial till strata is completely gone while the underlying and easily eroded lake bed sand strata is still there with no significant erosion other than where little cottonwood creek flows during the bonneville deep water surging this section of glacial till delta broke free and slid out into the depths of lake bonville second the case of the little willow fan scarps looking at the utah gs draper map we see the mapped scarps at the mouth of little willow canyon tend to fan out going east this would be very strange for grab-and-scarps but completely consistent with lake surging creating an eddy in dry creek cove and rotating the little willow delta out of place finally the bell canyon flood when event tea occurred the glaciers were receding there would have been a moraine lake at the base of belle's canyon when the marines split in a single catastrophic event the moraine lake would have flushed out through the rapidly forming fissure it would have resisted the sharp bend in the fissure and flowed out over the lower face the lower face shows evidence of this flood the features are similar to the missoula flood landscape features in a second image this time from the north the flood debris wall is distinctly apparent this all leads us to something very important the bells canyon flood debris field drops down through any reasonable bonneville lake level projection for the time of event t a loose debris field of this type of flood would be very susceptible to lakeshore erosion even over modest periods of time there is no discernible lakeshore in this field or in any of the other gilbert trough features or earthquake scarp features identified the inescapable conclusion is that immediately following the event t earthquake the lake level started the rapid drop of the bonneville flood this is red rock pass in southeastern idaho at bonneville's high stand the outflow here was stable for thousands of years of changing climate and hundred year storms then it failed in one catastrophic event the event t earthquake may have been such an event particularly if it occurred in concert with slips in other adjacent fault segments the west valley fault in the salt lake valley slipped sometime in that same time period i think of level evidence from other parts of lake bonneville is re-examined it will be found to be consistent with surging and tsunami at the time of event t i hope you have enjoyed this geological murder mystery the gilbert trough is a fisher not a graven and the bonneville flood needs to be re-examined in light of this new information it must have been a spectacular day in the wasatch [Music] you

Channel: Bonneville Flood and the Wasatch Fault

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Length: 12min 24sec (744 seconds)

Published: Tue Feb 02 2021