Position2D

(* http://stackoverflow.com/questions/8364804 *)

Position2D[bigMat_?MatrixQ, smallMat_?MatrixQ] :=
 Module[{pos, sdim = Dimensions[smallMat] - 1},
  pos = Position[bigMat, smallMat[[1, 1]]];
  Quiet[Select[pos, (MatchQ[
      bigMat[[Sequence@@Thread[Span[#, # + sdim]]]], smallMat] &)],
    Part::take]]

(* Example *)

Position2D[{{0, 1, 2, 3}, {1, 2, 3, 4}, {2, 3, 4, 5}, {3, 5, 5, 6}},
   {{2, 3}, {3, _}}]

(* Returns: {{1, 3}, {2, 2}, {3, 1}} *)