gistfile1.txt

First generate a list of the timestamps of the I-frames:

mkvinfo -s input.mkv | grep track\ 1 | grep I\ frame | grep -o -E "[0-9]{1,2}\:[0-9]{1,2}\:[0-9]{1,2}\.[0-9]{3}" > I-frames_stripped.txt

Now copy the timestamps you want into another file.
Then you can run:

mkvmerge --split timecodes:$(cat marks.txt) input.mkv -o input_split.mkv

where marks.txt contains your timestamps.

You can then use:

mkvmerge -o combined.mkv input_split-001.mkv + input_split-002.mkv + input_split-003.mkv [...]

to combine your videofiles.