What happens if the usual geodesic equation on an n-manifold is directly modified from a source dimension 1 space (giving a path) to a dimension 2 space (giving a surface). I suspect that this gives a totally geodesic surface, but I was hoping for somewhere to discuss modifications (adding extra terms like a connection on the 2D source space) and the integrability conditions quite explicitly.

This must be well known in differential geometry, just not to me as a non-expert, so please give a reference rather than just down voting.