CSX Corp. directors unanimously approved a merger and stock-purchase plan yesterday with the Owensboro, Ky.-based Texas Gas Resources Corp. Texas Gas Resources directors already have approved the agreement.

A statement issued by CSX said it would start a cash tender offer Thursday for 7.1 million Texas Gas shares--about 35 percent of the outstanding shares--at $52 a share net to the seller.

CSX also said it has reserved rights to buy up to 3.2 million additional Texas Gas shares--about 16 percent of the outstanding shares--under the tender offer, and agreed to purchase another 3.74 million newly issued Texas Gas shares.

CSX is a transportation and natural resources holding company whose major units are the Chessie System and Seaboard railroads. It has assets of $8.1 billion.

Texas Gas, with assets of $1.8 billion, holds major interests in gas transmission, oil and gas exploration and production, and inland waterways barge and shipbuilding.

The CSX board also approved a three-for-one split of the corporation's common stock, subject to shareholder approval, which will be sought at a special meeting in which the Texas Gas merger will be submitted for approval. No date was announced.