Almost looks like Phobos originally was a large chunk of sedimentary rock.

'et' says: fractures from the Stickney impact... but those would be radial, not parallel, wouldn't they?

There are two things I'm reminded of, staring at that photo:

[1] The grooves, if that's what they are, bear a strong resemblance to the tracks left by rolling boulders. Someone, I forget who, posted an MGS image a couple of weeks or so showing just such tracks on the Martian surface - some even had the boulders at their termini.

What would make tracks like that on Phobos? I haven't a clue. There seem not to be any boulders - the "tracks" turn the corner at the lower right, leading one to surmise they continue for a while. Maybe... fair sized chunks from the Stickney impact, or other lesser (later?) events, returned to the surface at a very low angle and rolled instead of cratering. My guess is based on Phobos' very low gravity allowing a low-angle, low-velocity approach.

[2] Not all layered terrain is sedimentary. I just spent three months in a location where some of the exposed rock is alternating layers of basalt and granite. The basalt was even shot through with bubbles, indicating (to me) a volcanic origin. If Phobos is captured impact ejecta from Mars, rather than a captured planetesimal, could it have retained possible layered characteristics? Could such a piece of surface rock be given orbital velocity without melting and attaining a more spherical shape while cooling?