<PAGE>   1
                                                                   EXHIBIT 4.(o)


================================================================================


                                   MAPCO INC.

                                      and

                 THE FIRST NATIONAL BANK OF CHICAGO, as Trustee


                   ---------------------------------------


                          SUPPLEMENTAL INDENTURE NO. 1

                              Dated March 5, 1997


                   ---------------------------------------




                                   Series of
                              7.25% Notes due 2009
                                  $100,000,000

================================================================================

<PAGE>   2
                                   MAPCO INC.

                          SUPPLEMENTAL INDENTURE NO. 1

                                  $100,000,000
                              7.25% Notes due 2009


                 SUPPLEMENTAL INDENTURE NO. 1, dated as of March 5, 1997,
between MAPCO INC., a Delaware corporation (the "Company"), and THE FIRST
NATIONAL BANK OF CHICAGO, a national banking association, as trustee (the
"Trustee").


                                    Recitals


                 The Company has heretofore executed and delivered to the
Trustee a Senior Indenture, dated as of February 25, 1997 (the "Indenture"),
providing for the issuance from time to time of a series of the Company's
Securities.

                 Section 3.1 of the Indenture provides for various matters with
respect to any series of Securities issued under the Indenture to be
established in an indenture supplemental to the Indenture.

                 Section 8.1(7) of the Indenture provides for the Company and
the Trustee to enter into an indenture supplemental to the Indenture to
establish the form or terms of Securities of any series as permitted by
Sections 2.1 and 3.1 of the Indenture.

                 For and in consideration of the premises and the issuance of
the series of Securities provided for herein, it is mutually covenanted and
agreed as follows for the equal and ratable benefit of the Holders of the
Securities of such series:
<PAGE>   3
                                   ARTICLE 1

                       Relation to Indenture; Definitions

                 Section 1.1.  This Supplemental Indenture No. 1 constitutes an
integral part of the Indenture.

                 Section 1.2.  For all purposes of this Supplemental Indenture
No. 1:

                 (1)  Capitalized terms used herein without definition shall
         have the meanings specified in the Indenture;

                 (2)  all references herein to Articles and Sections, unless
         otherwise specified, refer to the corresponding Articles and Sections
         of this Supplemental Indenture No. 1; and

                 (3)  the terms "herein", "hereof", "hereunder" and other words
         of similar import refer to this Supplemental Indenture No. 1.


                                   ARTICLE 2

                              The Series of Notes

                 Section 2.1.  Title of the Securities.  There shall be a series
of Securities designated as the "7.25% Notes due 2009" (the "Notes").

                 Section 2.2.  Limitation on Aggregate Principal Amount; Date of
Notes.  The aggregate principal amount of the Notes shall not exceed
$100,000,000.  Each Note shall be dated the date of its authentication.

                 Section 2.3.  Principal Payment Date.  The principal of the
Notes shall be payable in a single installment on March 1, 2009.



                                      2
<PAGE>   4
                 Section 2.4.  Interest and Interest Rates.  Interest on the
Notes shall be payable semi-annually on March 1 and September 1 of each year
beginning on September 1, 1997 (each, an "Interest Payment Date"); provided,
however, that if an Interest Payment Date would otherwise be a day that is not
a Business Day, such Interest Payment Date shall be the succeeding Business
Day.  The interest so payable on any Note which is punctually paid or duly
provided for on any Interest Payment Date shall be paid to the Person in whose
name such Note is registered at the close of business on the February 15 or
August 15, respectively (whether or not a Business Day), preceding such
Interest Payment Date (each, a "Regular Record Date").

                 The interest payable on each Interest Payment Date shall be
the amount of interest accrued for the period from and including March 5, 1997
or from and including the most recent Interest Payment Date to which interest
has been paid or duly provided for, as the case may be, to, but excluding, such
Interest Payment Date.  Interest shall be computed on the basis of a 360-day
year consisting of twelve 30-day months.

                 The interest rate borne by the Notes will be 7.25% per annum
until paid in full.

                 Section 2.5.  Place of Payment.  The Place of Payment where the
Notes may be presented or surrendered for payment, where the principal of and
interest and any other payments due on the Notes are payable, where the Notes
may be surrendered for registration of transfer or exchange and where notices
and demands to and upon the Company in respect of the Notes and the Indenture
may be served shall be in the Borough of Manhattan, The City of New York, and
the office or agency maintained by the Company for such purpose shall initially
be the Corporate Trust Office of the Trustee.

                 Section 2.6.  Redemption.  The Notes shall not be subject to
redemption at the option of the Company at any time prior to their Stated
Maturity, and the Company shall


                                      3
<PAGE>   5
have no obligation to redeem or purchase the Notes pursuant to any sinking fund
or analogous provisions or upon the happening of any specified event or at the
option of any Holder of the Notes.

                 Section 2.7.  Denomination.  The Notes shall be issued in
denominations of $1,000 and integral multiples thereof.

                 Section 2.8.  Currency.  Principal and interest on the Notes
shall be payable in Dollars.

                 Section 2.9.  Registered Securities.  The Notes shall be issued
as Registered Securities, without coupons.

                 Section 2.10.  Notes to be Issued in Global Form; Exchange for
Certificated Notes.  The Notes will be initially represented by one or more
Notes in global form (the "Global Notes").  The Company hereby designates The
Depository Trust Company as the initial Depositary for the Global Notes.  The
Global Notes will be deposited with the Trustee, as custodian for the
Depositary.  Unless and until they are exchanged in whole or in part for Notes
in certificated form, the Global Notes may not be transferred except as a whole
by the Depositary to a nominee of the Depositary or by a nominee of the
Depositary to the Depositary or another nominee of the Depositary or by the
Depositary or any such nominee to a successor Depositary for the Notes or a
nominee of such successor Depositary.  The Depositary may surrender a Global
Note in exchange in whole or in part for Notes in certificated form on such
terms as are acceptable to the Company and the Depositary.

                 The Company may at any time in its sole discretion determine
that all or any portion of the Notes shall no longer be represented by a Note
or Notes in global form.  In such event the Company shall execute, and the
Trustee, upon receipt of a Company Order for the authentication and delivery of
certificated Notes of like tenor, shall authenticate and deliver Notes of like
tenor in certificated




                                       4
<PAGE>   6
form, in authorized denominations and in an aggregate principal amount equal to
the applicable principal amount of the Global Note or Global Notes, in exchange
for such Global Note or Global Notes (or the applicable portion thereof).

                 Section 2.11.  Form of Notes.  The Notes shall be substantially
in the form attached as Exhibit A hereto.

                 Section 2.12.  Defeasance and Covenant Defeasance.  The
provisions of Sections 4.4 and 4.5 of the Indenture shall apply to the Notes.

                 Section 2.13.  Registrar and Paying Agent.  The Trustee shall
initially serve as Registrar and Paying Agent.

                                   ARTICLE 3

                            Miscellaneous Provisions

                 Section 3.1.  The Indenture, as supplemented and amended by
this Supplemental Indenture No. 1, is in all respects hereby adopted, ratified
and confirmed.

                 Section 3.2.  This Supplemental Indenture No. 1 may be executed
in any number of counterparts, each of which when so executed shall be deemed
an original; and all such counterparts shall together constitute but one and
the same instrument.

                 SECTION 3.3.  THIS SUPPLEMENTAL INDENTURE NO. 1 AND EACH NOTE
SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF
NEW YORK.



                                      5
<PAGE>   7
                 IN WITNESS WHEREOF, the parties hereto have caused this
Supplemental Indenture No. 1 to be duly executed, as of the day and year first
written above.

                                           MAPCO INC.



                                           By /s/ PHILIP W. BAXTER
                                             ----------------------------------
                                             Name:  Philip W. Baxter
                                             Title: Executive Vice President
                                                    Officer and Chief Financial

[Corporate Seal]

ATTEST:


By  /s/ JAMES N. CUNDIFF
  -------------------------------
  Name:  James N. Cundiff
  Title: Assistant Secretary

                                           THE FIRST NATIONAL BANK OF
                                             CHICAGO, Trustee


                                           By /s/ JOHN R. PRENDIVILLE
                                             ----------------------------------
                                             Name:  John R. Prendiville
                                             Title: Vice President

[Seal]

ATTEST:


By  /s/ JEFFREY L. KINNEY
  -------------------------------
  Name:  Jeffrey L. Kinney
  Title: Assistant Vice President



                                      6
<PAGE>   8
                                                                       EXHIBIT A





                             [FORM OF FACE OF NOTE]

                 THIS NOTE IS IN GLOBAL FORM WITHIN THE MEANING OF THE
         INDENTURE HEREINAFTER REFERRED TO AND IS REGISTERED IN THE NAME OF A
         DEPOSITARY OR A NOMINEE OF A DEPOSITARY.  UNLESS AND UNTIL IT IS
         EXCHANGED IN WHOLE OR IN PART FOR NOTES IN CERTIFICATED FORM, THIS
         NOTE MAY NOT BE TRANSFERRED EXCEPT AS A WHOLE BY THE DEPOSITARY TO A
         NOMINEE OF THE DEPOSITARY OR BY A NOMINEE OF THE DEPOSITARY TO THE
         DEPOSITARY OR ANOTHER NOMINEE OF THE DEPOSITARY OR BY THE DEPOSITARY
         OR ANY SUCH NOMINEE TO A SUCCESSOR DEPOSITARY OR A NOMINEE OF SUCH
         SUCCESSOR DEPOSITARY.

                                   MAPCO INC.

                              7.25% Notes due 2009


                                                           CUSIP No. 565097 AG 7

No. 1
                                                                    $100,000,000


                 MAPCO Inc., a corporation duly organized and existing under
the laws of the State of Delaware (herein called the "Company", which term
includes any successor corporation under the Indenture referred to herein), for
value received, hereby promises to pay to Cede & Co, or registered assigns, the
principal sum of one hundred million Dollars (U.S. $100,000,000) on March 1,
2009, and to pay on March 1 and September 1 of each year beginning on September
1, 1997 (each, an "Interest Payment Date"; provided, however, that if an
Interest Payment Date would otherwise be a day that is not a Business Day, such
Interest Payment Date shall be the succeeding Business Day) the amount of
interest accrued thereon for the period from and including March 5, 1997 or
from and including the most recent Interest Payment Date to which interest has
been paid or duly provided for,
<PAGE>   9

as the case may be, to, but excluding, such Interest Payment Date, at the rate
of 7.25% per annum.  The rate of interest on any overdue principal and, to the
extent permitted by applicable law, overdue interest shall be the rate of
interest in effect on this Note from time to time.  Interest shall be computed
on the basis of a 360-day year consisting of twelve 30-day months.  The
interest so payable on any Interest Payment Date which is punctually paid or
duly provided for on such Interest Payment Date will, as provided in the
Indenture referred to on the reverse hereof, be paid to the Person in whose
name this Note is registered at the close of business on the Regular Record
Date for such interest, which shall be the February 15 or August 15,
respectively (whether or not a Business Day), preceding such Interest Payment
Date.  Interest payable on this Note which is not punctually paid or duly
provided for on any Interest Payment Date therefor shall forthwith cease to be
payable to the Person in whose name this Note is registered at the close of
business on the Regular Record Date preceding such Interest Payment Date, and
such defaulted interest shall instead be payable to the Person in whose name
this Note is registered on the special record date or other specified date
determined in accordance with the Indenture referred to on the reverse hereof.

                 Payment of the principal of and interest on this Note will be
made at the office or agency of the Company maintained for that purpose in the
Borough of Manhattan, The City of New York (which shall initially be an office
or agency of the Trustee), in such coin or currency of the United States of
America as at the time of payment is legal tender for payment of public and
private debts; provided, however, that at the option of the Company, interest
on the Notes may be paid (i) by check mailed to the address of the Person
entitled thereto as such address shall appear in the register of Holders of the
Notes or (ii) at the expense of the Company, by wire transfer to an account
maintained by the Person entitled thereto as specified in the register of
Holders of the Notes.



                                      2
<PAGE>   10
                 Reference is hereby made to the further provisions of this
Note set forth on the reverse hereof, which further provisions shall for all
purposes have the same effect as if set forth at this place.

                 Unless the certificate of authentication hereon has been
executed by the Trustee referred to on the reverse hereof by manual signature,
this Note shall not be entitled to any benefit under the Indenture referred to
on the reverse hereof or be valid or obligatory for any purpose.

                 IN WITNESS WHEREOF, MAPCO Inc. has caused this instrument to
be duly executed under its corporate seal.


Dated:

[Corporate Seal]                       MAPCO INC.



                                       By:                           
                                          ------------------------------------
                                          Title:


               [FORM OF TRUSTEE'S CERTIFICATE OF AUTHENTICATION]

          This is one of the Securities of a series described in the
within-mentioned Indenture.


                                       THE FIRST NATIONAL BANK OF
                                          CHICAGO, as Trustee



                                       By:                           
                                          ------------------------------------
                                          Authorized Signatory


                                      3
<PAGE>   11
                           [FORM OF REVERSE OF NOTE]


                 This Note is one of a duly authorized issue of securities of
the Company (herein called the "Securities"), issued and to be issued in one or
more series under an Indenture, dated as of February 25, 1997 (herein called
the "Indenture"), between the Company and The First National Bank of Chicago
(herein called the "Trustee", which term includes any successor trustee under
the Indenture), to which Indenture and all indentures supplemental thereto
reference is hereby made for a statement of the respective rights, limitations
of rights, duties and immunities thereunder of the Company, the Trustee and the
Holders of the Securities and of the terms upon which the Securities are, and
are to be, authenticated and delivered.  This Note is one of the series
designated on the face hereof (collectively, the "Notes"), limited to
$100,000,000 in aggregate principal amount, and is issued pursuant to
Supplemental Indenture No. 1, dated March 5, 1997, between the Company and the
Trustee, relating to the Notes.

                 The Notes shall not be subject to redemption at the option of
the Company at any time and the Company shall have no obligation to redeem or
purchase the Notes pursuant to any sinking fund or upon the happening of any
specified event or at the option of any Holder of the Notes.

                 The Indenture contains provisions for defeasance and covenant
defeasance at any time of the indebtedness on this Note upon compliance by the
Company with certain conditions set forth therein, which provisions apply to
this Note.

                 If an Event of Default with respect to the Notes shall occur
and be continuing, the principal of the Notes may be declared due and payable
in the manner and with the effect provided in the Indenture.
<PAGE>   12
                 The Indenture permits, with certain exceptions as therein
provided, the amendment thereof and the modification of the rights and
obligations of the Company and the rights of the Holders of the Securities of
each series to be affected under the Indenture at any time by the Company and
the Trustee with the consent of the Holders of not less than a majority in
aggregate principal amount of the Securities at the time Outstanding of all
series adversely affected (voting as one class).  The Indenture also contains
provisions permitting the Holders of specified percentages in principal amount
of the Securities of each series at the time Outstanding, on behalf of the
Holders of all Securities of such series, to waive past Defaults or Events of
Default under the Indenture and their consequences.  Any such consent or waiver
by the Holder of this Note shall be conclusive and binding upon such Holder and
upon all future Holders of this Note and of any Note issued upon the
registration of transfer hereof or in exchange herefor or in lieu hereof,
whether or not notation of such consent or waiver is made upon this Note.

                 No reference herein to the Indenture and no provision of this
Note or of the Indenture shall alter or impair the obligation of the Company,
which is absolute and unconditional, to pay the principal of, and interest on,
this Note at the times, place and rate, and in the coin or currency, herein
prescribed.

                 As provided in the Indenture and subject to certain
limitations as therein set forth, the transfer of this Note is registrable on
the Register, upon surrender of this Note for registration of transfer at the
office or agency of the Company to be maintained for that purpose in The City
of New York, duly endorsed by, or accompanied by a written instrument of
transfer in form satisfactory to the Company, the Trustee and the Registrar
duly executed by, the Holder hereof or his attorney duly authorized in writing,
and thereupon one or more new Notes, of like tenor, of authorized denominations
and for the same aggregate principal amount, will be issued to the designated
transferee or transferees.


                                      2
<PAGE>   13
                 The Notes are issuable only in registered form in
denominations of $1,000 and any integral multiple thereof.  As provided in the
Indenture and subject to certain limitations therein set forth, the Notes are
exchangeable for Notes having a like aggregate principal amount, of like tenor
and having a different authorized denomination, as requested by the Holder
surrendering the same.

                 No service charge shall be made for any such registration of
transfer or exchange, but the Company may require payment of a sum sufficient
to cover any tax or other governmental charge that may be imposed in connection
therewith.

                 Prior to due presentment of this Note for registration of
transfer, the Company, the Trustee and any agent of the Company or the Trustee
may treat the Person in whose name this Note is registered as the owner hereof
for the purpose of receiving payment of principal of and (subject to Section
3.7 of the Indenture) interest on this Note and for all other purposes
whatsoever, whether or not this Note shall be overdue, and neither the Company,
the Trustee nor any agent of the Company or the Trustee shall be affected by
notice to the contrary.

                 Capitalized terms used in this Note without definition shall
have the meanings specified in the Indenture.


                                      3