UNAWARE IT IS CLOSED; C208 FLT CREW CIRCLES TO AND LANDS ON RWY 23 AT DNL.

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PERFORMED TOUCH AND GO ON CLOSED RWY AT DNL. FLEW GPS-B; CIRCLE TO LAND RWY 23 OPTION BACK TO RADAR FOR ANOTHER INST APCH. CHOSE TO LAND RWY 23 SINCE IT WAS LONGEST AND WINDS WERE CALM. SWITCHED FROM AUGUSTA APCH FREQ TO UNICOM AFTER RECEIVING OUR CLBOUT INSTRUCTIONS AND ADVISED TO CHANGE AT APPROX 3 NM FROM FIELD. WE INITIATED OUR CIRCLING APCH AND A CALL WAS MADE ON UNICOM TO ADVISE OF OUR INTENTION TO CIRCLE FOR A R BASE TO RWY 23. WE SAW CRANES R OF CTRLINE ON FINAL AND AIMED TO TOUCH DOWN PAST THE DISPLACED THRESHOLD. WE COMPLETED TOUCH AND GO LNDG UNEVENTFULLY; AND ON CLBOUT THE CAPT (PNF) OBSERVED AN 'X' ON DEP END. AFTER TURNOUT I ALSO SAW THE 'X' ON DEP END. WE CONTACTED AUGUSTA APCH ON CLBOUT AND ASKED IF RWY 5/23 WAS CLOSED. HE ANSWERED AFFIRMATIVE. WE ADVISED AUGUSTA APCH CTL THAT WE PERFORMED A TOUCH AND GO ON CLOSED RWY SINCE IT WAS NOT IN OUR COMPANY NOTAMS. WE FLEW ANOTHER GPS-B AND CIRCLED TO RWY 11; FULL STOP UNEVENTFULLY. AFTER TAXIING IN; WE SAW THAT AN 'X' WAS MARKED IN THE OVERRUN OF RWY 23. NEITHER PLT SAW 'X' ON APCH END OF RWY 23 DURING TOUCH AND GO. I WAS FOCUSING ON MY TOUCHDOWN POINT BEYOND DISPLACED THRESHOLD AND DID NOT SEE 'X.' FURTHERMORE; THE NOTAMS PROVIDED BY OUR COMPANY DID NOT INCLUDE CLOSED RWY.