S56 CTLR DESCRIBED OPERROR WHEN SEPARATION WAS LOST BTWN 2 ACFT AND CLRNC FROM TERRAIN WAS LESS THAN REQUIRED.

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I WAS WORKING THE ARR FEEDER SECTOR. IT WAS BUSY WITH WX IN THE AREA AND STRONG WINDS FROM THE SW. WE WERE SUPPOSED TO BE GETTING MI IN TRAIL FROM ZLC; OVER JAMNN 20 MI AND OVER SPAIN 12 MI. WE WERE NOT GETTING THE REQUIRED SPACING FROM ZLC WHICH REQUIRED EXCESSIVE VECTORING TO ACHIEVE SPACING TO THE FINAL CTLR AND THE COORD DOWNWIND SPD OF 190 KTS. ACR X WAS ON THE SPAIN ARR AND WAS VECTORED OFF FOR SEQUENCING. ACR Y WAS ON THE JAMNN ARR SUPPOSED TO BE 20 MI IN TRAIL WITH THE ACFT AHEAD AND WAS ONLY 15 MI IN TRAIL. ACR Y WAS SLOWED ON INITIAL CONTACT TO 210 KTS AND DSNDED TO 13000 FT. ACR X WAS BEING VECTORED ACROSS ACR Y'S FLT PATH AND ACR Y WAS TURNED E FOR ADDITIONAL SPACING. ACR X WAS DSNDED; I THOUGHT; TO 13000 FT AND TURNED N AHEAD OF ACR Y. I OBSERVED ACR X DSNDING BELOW 13000 FT; THE MVA FOR THE AREA HE WAS AT. I ASKED HIM TO VERIFY ALT AND TO MAINTAIN 13000 FT. HE SAID HE WAS ASSIGNED 11000 FT AND WAS CLBING BACK UP. I OBSERVED THE MODE C OF ACR X AT 12400 FT IN THE 12600 FT MVA AND ADVISED MY SUPVR OF A POSSIBLE LOSS OF SEPARATION. THE INITIAL INVESTIGATION INDICATES THAT ACR X DSNDED TO 12200 FT IN THE 12600 FT MVA AND THAT ACR Y; A B737; WAS VECTORED 4.7 MI BEING ACR X; A HVY; AND THE MINIMUM SEPARATION IS 5 MI.