by: Charles O’Dale
- 1. introduction
- 2. specific crater rims
- Bruno (lunar)
- St Martin
At deeper levels of the impact where the material is not ejected, tensional stresses in the release waves are lower. As a result, fracturing is less pronounced, excavation flow velocities are lower, and the excavation flow lines themselves are not oriented to eject material beyond the crater rim. This region forms a displaced zone in which material is driven downward and outward more or less coherently. Both zones in the transient crater continue to expand, accompanied by the uplift of near-surface rocks to form the transient crater rim. However, these waves continually loose energy by deforming and ejecting the target rocks through which they pass. Eventually, a point is reached at which the shock and release waves can no longer excavate or displace target rock. At that point the growth of the transient crater ceases. (French, 1998).
2. SPECIFIC CRATER RIM
BRUNO – LUNAR
It is estimated that it was a 1-3 km wide asteroid that impacted at the Moon’s northeast limb to form the 22 km diameter Giordano Bruno Crater. When viewed from orbit, Giordano Bruno is at the center of a symmetrical ray system of ejecta that has a higher albedo than the surrounding surface, implying a high angle impact. The ray material extends for over 150 kilometers and has not been significantly darkened by space erosion.
As illustrated at the Giordano Bruno Crater, what remains when the growth of the transient crater stops is a depression with an upraised rim, and the modification stage begins. The exposed rim, walls, and floor define the so-called apparent crater. At the rim, there is an overturned flap of ejected target materials, which displays inverted stratigraphy, with respect to the original target materials (Grieve, 2002). An overturned rim sequence is now recognized as one of the hallmarks of an impact crater.
The rim of the 1.19 kilometre diameter Barringer Crater is still well defined, even after approximately 49 thousand years of erosion. It has been estimated that the first two stages of the cratering process (time from initial contact of the impactor until the end of the excavation stage) here at Barringer took approximately 6 seconds! Almost 63 million cubic metres were evacuated from this area in that time to form the crater. The height of the rim over the surrounding plain is 36 – 61 metres. Investigations around this rim confirmed an “overturned rim sequence”.
My 11 kilometre exploration hike around the rim of the 3.44 kilometre diameter Pingualuit Impact Structure took most of a day and was not one of the easiest of hikes that I have experienced. Along the lip of the rim there were frequent gullies that we had to traverse. This image gives you a good size perspective, as the people leading the hike are just visible on the rim in the far distance.
The rim of the St. Martin complex crater is buried by over 100 metres of Jurassic red beds and glacial drift. It is my hypothesis that the cause of this extreme diversion of the Dauphin River at the rim of the St. Martin crater is the differential sagging of the outlaying bedrock compared to the breccia within the impact structure. To my knowledge, there is no published report that explains the cause of this river’s diversion at this specific location. The Dauphin River then follows this rim to the East and flows into Lake Winnipeg. My hypothesis alleges this phenomena is analogous to the James and York River diversions within the Chesapeake Impact Structure.