Hi Lynn,
I have been researching your question about entry of 'texture' for an M layer in NASIS. I found nothing specific to answer your question in SSM, NSSH, nor NASIS user guides. But by definition, the M layer is a continuous human-manufactured layer, so as such, would not receive a soil texture designation. It could be named, in this case asphalt, perhaps entered in the in-lieu of texture. It is continuous, consolidated/cemented(?), and root restrictive. Maybe someone else who works in these materials with some frequency can pipe in and provide their solution.