The intrusive igneous rock body you're describing is called a sill.

Here's a breakdown of the terms:

* Intrusive: The igneous rock formed from magma that cooled and solidified beneath the Earth's surface.

* Tabular: The rock body is flat and sheet-like.

* Concordant: The rock body is parallel to the layering of the surrounding sedimentary rocks.

Sills are formed when magma intrudes into existing sedimentary layers, forcing them apart. They are often horizontal, but can also be inclined or even vertical in some cases.