Complex sentence: Above that would be a stratum of silicon carbide and titanium carbide, and a layer of carbon above that in the form of graphite, possibly with a layer of diamond at the bottom, kilometers thick, if there is sufficient pressure.
Split into simpler sentences: Above that would be a thick layer of silicon carbide and titanium carbide, and a layer of carbon above that. The carbon would be in the form of graphite, possibly with a layer of diamond at the bottom, kilometers thick, if there is sufficient pressure.