Younger Dryas Impact Hypothesis: Part 2

So, following on from my last post, what are some other potential issues with the Younger Dryas Impact Hypothesis? 

Craters

There are no well-dated craters from the end-Pleistocene. However, the impact proponents have argued for: 

A) an impact on the Laurentide ice sheet, which didn't leave a crater due to hitting the ice, or 

B) an airburst explosion that affected the whole continent (and beyond) and left no craters, or 

C) a combination of the two.

The Carolina Bays and playa basins of the Great Plains have been suggested as evidence for an impact around 12,900 BP (Holliday et al., 2014). The bays are elliptical depressions found along the Atlantic Coastal Plain, North America. However, dating now indicates that the bays formed over a period of time throughout the late Pleistocene, but before 12,900 BP (Holliday et al., 2014).

The playas are smaller, and contain water at some times of year. There are over 20,000 of these, and only one has been confirmed as the result of an impact. All others have a roughly horizontal erosional unconformity (a break in the rock layer succession as result of erosion wearing down top layers. Over time, new layers are deposited on top. The surface where these contact is the unconformity) between the playa layer and the older, underlying layers. This is not the geology of an impact site, it's the result of terrestrial geomorphic processes (Holliday et al., 2014).

Ice sheet impact

The impactor may have broken up to produce a scattering of airbursts, starting wildfires across the continent and destabilising the ice sheet (without leaving any crater/s). However, a 4 km wide comet (the size calculated to be needed to cause the widespread environmental changes Firestone et al. (2007) hitting an ice sheet, would shock the rock layers beneath the ice, leaving behind an impact structure of some kind but the record of landforms and sediments left at the ice margin provide no evidence for an impact around 12,900 BP (Holliday et al., 2014).

From: Quaternary Geology
Laurentide Ice Sheet: the larger ice sheet on the right. The one on the left is the Cordilleran Ice Sheet, with the ice free corridor between. This corridor is one of the hypothesised routes of humans into North America

Holliday et al., (2014) state 'the basic physics of the YDIH does not agree with the physics of impacts nor the basic laws of physics'. (Ouch!) The mechanisms suggested for the explosion do not conserve energy or momentum (Holliday et al., 2014). On top of this, they argue that no mechanism is known to create an airburst that would affect an entire continent. 

[This may seem like gobbledygook but I’m sure the physics readers will be nodding along, (at least I hope so). As a non-physicist, you may have to take a leap of faith here along with me: conserving energy and momentum are important laws that things like comets normally are expected to follow.]  

The rest of this argument will be in the final Part 3, since the last part is actually the most interesting in my opinion and it'd be a shame if you were asleep by the time you got there...