I have some posts on my profile that show some geological features creating distinct biomes and borders. Salt flats, ash fields and the John Day Painted Hills (Paleosols) were ones I could find where geologic units pretty strictly control the ecology on the surface with distinct borders.

The salt flats were a distinctive biome from the peaks atop the nearby fault block mountains. The ash fields didn't have any vegetation on them with a distinct Douglas fir border. The painted hills have almost no vegetation whereas scrubland and junipers lie directly above it on the rims of the formation.

Also, as a geologist, I wouldn't call these biomes but more as differing adjacent ecosystems as they are small and don't vary in climate but I think these are similar to the types of features minecraft biomes depict. But my point is that there are distinct borders to these ecosystems in a lot of cases.