Anyone who thinks that a new trade deal between the UK and US is going to assist with avoiding market carnage is mistaken. Trade between the UK and US is insignificant compared to trade with the EU. No US trade deal can fill the impending Brexit void.
The UK trades 4.4x as many goods with the EU as the US. Distance is key to trade volumes. Even if all UK-US trade barriers removed — which sounds great — it could only make up for a moderate fraction of the trade lost with the EU in a no-deal Brexit.