Those memories just before and just after the impact are lost because they did not get stored into memory. They were in immediate memory which is volatile (easily lost).

It is like you had a flood of cleaning chemicals coursing through your brain from the impact. It washed away the memories from just before the impact. It was still flowing through your brain for a while after the impact so it kept washing those post traumatic memories away. Once your brain settled down and shut off the flow of cleaning chemicals, your memories started to get saved into long term memory banks.

I hope this makes sense.

I doubt you will get those small bits of memory back. Consider it a blessing that you do not remember the trauma.

My best to you.