<PAGE>
                                                                   EXHIBIT 4.2.1


                          SIXTH SUPPLEMENTAL INDENTURE
              TO INDENTURE DATED MARCH 15, 1997 (8 1/2% SECURITIES)


               SIXTH SUPPLEMENTAL INDENTURE dated as of December 31, 1999, among
CHESAPEAKE ENERGY CORPORATION, an Oklahoma corporation (the "Company"), the
SUBSIDIARY GUARANTORS listed as signatories hereto, and UNITED STATES TRUST
COMPANY OF NEW YORK, a New York corporation, as Trustee to the Indenture (the
"Trustee").

               WHEREAS, Chesapeake Mid-Continent Corp., an Oklahoma corporation
("CMCC"), is a Restricted Subsidiary of the Company and a Subsidiary Guarantor
under the Indenture, and CMCC has directly merged with and into Chesapeake
Exploration Limited Partnership, an Oklahoma limited partnership ("CELP"), and
CELP is the surviving entity, a Restricted Subsidiary of the Company and a
Subsidiary Guarantor under the Indenture;

               WHEREAS, Section 10.2(a) of the Indenture provides, among other
things, that no Subsidiary Guarantor may consolidate or merge with or into
another corporation, entity or Person unless (i) the entity or Person formed by
or surviving such consolidation or merger (if other than such Subsidiary
Guarantor) assumes all the obligations of such Subsidiary Guarantor pursuant to
a supplemental indenture, in a form reasonably satisfactory to the Trustee,
under the Securities and the Indenture and (ii) immediately after such
transaction, no Default or Event of Default exists;

               WHEREAS, no Default or Event of Default exists immediately after
the merger of CMCC into CELP;

               WHEREAS, the form and substance of this Sixth Supplemental
Indenture are satisfactory to the Trustee;

               WHEREAS, contemporaneously herewith, there are being delivered to
the Trustee executed opinions of counsel and officers' certificate's proper in
form and substance;

               WHEREAS, Section 9.1 of the Indenture provides, among other
things, that the Trustee, the Subsidiary Guarantors and the Company may amend or
supplement the Indenture without notice to or consent of any Holder to reflect
the addition or release of any Subsidiary Guarantor, as provided for by the
Indenture; and

               WHEREAS, the execution and delivery of this Sixth Supplemental
Indenture have been duly authorized by the Company and the Subsidiary Guarantors
and all actions necessary to make this Sixth Supplemental Indenture a valid and
binding instrument according to its terms and the terms of the Original
Indenture have been performed.

               NOW, THEREFORE, BY THIS SIXTH SUPPLEMENTAL INDENTURE, for and in
consideration of the premises and of the mutual covenants herein contained and
for other



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valuable considerations, the receipt whereof is hereby acknowledged, the Company
and the Subsidiary Guarantors covenant and agree with the Trustee, for the equal
benefit of all present and future Holders of the Securities, as follows:

                                    ARTICLE I

                                   DEFINITIONS

               SECTION 1.1 The definitions set forth in or incorporated by
reference in Article I of the Indenture shall be applicable to this Sixth
Supplemental Indenture, as fully and to the same extent as if set forth herein,
except as otherwise expressly provided herein. As used in this Sixth
Supplemental Indenture, the following terms shall have the following meanings:

               "Indenture" means the Original Indenture, as amended by this
Sixth Supplemental Indenture, relating to the Securities.

               "Original Indenture" means the Indenture dated as of March 15,
1997, among the Company, the Subsidiary Guarantors listed as signatories thereto
and the Trustee, relating to the Securities, as amended by: (i) that certain
First Supplemental Indenture dated as of December 17, 1997, (ii) that certain
Second Supplemental Indenture dated as of February 16, 1998, (iii) that certain
Third Supplemental Indenture dated as of April 22, 1998, (iv) that certain
Fourth Supplemental Indenture dated as of July 1, 1998, and (v) that certain
Fifth Supplemental Indenture dated as of November 19, 1999.

                                   ARTICLE II

                         RELEASE OF SUBSIDIARY GUARANTOR

               SECTION 2.1 As a result of the direct merger with CELP, which
constitutes a merger with a Subsidiary Guarantor under Section 10.2(a) of the
Indenture, CMCC shall for all purposes be released as a Subsidiary Guarantor
from all of its Guarantee and related obligations in the Indenture, pursuant to
Section 10.4(b) of the Indenture.

               SECTION 2.2 The notation on the Securities relating to the
Guarantee shall be deemed to exclude the name of CMCC and the signature of an
Officer on behalf of CMCC.

                                   ARTICLE III

                            ASSUMPTION OF OBLIGATIONS

               SECTION 3.1 As the surviving entity in its merger with CMCC and
as a Subsidiary Guarantor, CELP hereby agrees to assume all the obligations of
CMCC.



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                                   ARTICLE IV

                                  MISCELLANEOUS

               SECTION 4.1 This Sixth Supplemental Indenture is a supplemental
indenture pursuant to Section 9.1 of the Indenture. Upon execution and delivery
of this Sixth Supplemental Indenture, the terms and conditions of this Sixth
Supplemental Indenture will be part of the terms and conditions of the Indenture
for any and all purposes, and all the terms and conditions of both shall be read
together as though they constitute one instrument, except that in case of
conflict, the provisions of this Sixth Supplemental Indenture will control.

               SECTION 4.2 Except as they have been modified in this Sixth
Supplemental Indenture, each and every term and provision of the Indenture shall
remain in full force and effect.

               SECTION 4.3 This Sixth Supplemental Indenture may be executed in
any number of counterparts, each of which when so executed and delivered shall
be an original, but such counterparts shall together constitute but one and the
same instrument.

               SECTION 4.4 This Sixth Supplemental Indenture shall be governed
by and construed in accordance with the laws of the State of New York without
giving effect to applicable principals of conflicts of law to the extent that
the application of the law of another jurisdiction would be required thereby.

                                   SIGNATURES

               IN WITNESS WHEREOF, the parties hereto have caused this Sixth
Supplemental Indenture to be duly executed as of the date first written above.


                                 CHESAPEAKE ENERGY CORPORATION, an
                                 Oklahoma corporation


                                 By  /s/ AUBREY K. MCCLENDON
                                   --------------------------------------------
                                   Aubrey K. McClendon, Chief Executive Officer


                                 UNITED STATES TRUST COMPANY OF NEW
                                 YORK, a New York corporation, as Trustee


                                 By   /s/ LOUIS P. YOUNG
                                   --------------------------------------------
                                   Name:   Louis P. Young
                                        ---------------------------------------
                                   Title:  Authorized Signer
                                         -------------------------------------



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                                 SUBSIDIARY GUARANTORS


                                 CHESAPEAKE OPERATING, INC., an Oklahoma
                                 corporation


                                 By   /s/ AUBREY K. MCCLENDON
                                    -------------------------------------------
                                    Aubrey K. McClendon, Chief Executive Officer


                                 CHESAPEAKE PANHANDLE LIMITED
                                 PARTNERSHIP, an Oklahoma limited partnership

                                 By Chesapeake Operating, Inc., an Oklahoma
                                    corporation, Sole General Partner


                                    By  /s/ AUBREY K. MCCLENDON
                                      -----------------------------------------
                                      Aubrey K. McClendon,
                                      Chief Executive Officer


                                 CHESAPEAKE ACQUISITION CORPORATION,
                                 an Oklahoma corporation


                                 By  /s/ AUBREY K. MCCLENDON
                                   --------------------------------------------
                                   Aubrey K. McClendon, President


                                 CHESAPEAKE ROYALTY COMPANY, an
                                 Oklahoma corporation


                                 By  /s/ AUBREY K. MCCLENDON
                                   --------------------------------------------
                                   Aubrey K. McClendon, Chief Executive Officer





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                                 CHESAPEAKE EXPLORATION LIMITED
                                 PARTNERSHIP, an Oklahoma limited partnership

                                 By  Chesapeake Operating, Inc., an Oklahoma
                                     corporation, Sole General Partner


                                     By /s/ AUBREY K. MCCLENDON
                                      -----------------------------------------
                                      Aubrey K. McClendon,
                                      Chief Executive Officer


                                 CHESAPEAKE ENERGY LOUISIANA
                                 CORPORATION, an Oklahoma corporation


                                 By /s/ AUBREY K. MCCLENDON
                                   --------------------------------------------
                                   Aubrey K. McClendon, Chief Executive Officer


                                 CHESAPEAKE CANADA CORPORATION, an
                                 Alberta, Canada corporation


                                 By /s/ AUBREY K. MCCLENDON
                                   --------------------------------------------
                                   Aubrey K. McClendon, Chief Executive Officer


                                 CHESAPEAKE LOUISIANA, L.P., an Oklahoma
                                 limited partnership

                                 By  Chesapeake Operating, Inc., an Oklahoma
                                     corporation, Sole General Partner


                                     By /s/ AUBREY K. MCCLENDON
                                       ----------------------------------------
                                       Aubrey K. McClendon,
                                       Chief Executive Officer

                                 THE AMES COMPANY, INC., an Oklahoma
                                 corporation


                                 By /s/ AUBREY K. MCCLENDON
                                   --------------------------------------------
                                   Aubrey K. McClendon, Chief Executive Officer



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