That is generally true for layered clays--a 2:1 layered clay has more surface area and generally more reactive sites than a 1:1 clay. It can also be about the types of minerals and the amount of potential reactive sites they have. For instance, amorphous minerals generally hold more OC than similar minerals with a more crystalline structure. Margaret has a good paper in Nature about this (1997). I would also look at papers by Katherine Heckman. In terms of with depth, many studies have found that a higher percentage of OC is mineral-associated at depth based on the amount of OC released when the samples are demineralized (look at papers from the early to mid 2000's from Cornelia Rumpel's lab).