Police said the accident happened between Junction 6 for Horwich and Bolton and Junction 8 for Chorley, Lancs.
The southbound section of the busy motorway has been completely closed to vehicles.
Traffic police shut just one lane in the immediate aftermath of the collision but later closed both carriageways beacuse of a spillage and damage to a central reservation barrier.
Drivers stuck behind the closure are now being slowly released from the area.
The road remains closed between Junction 8 and Junction 6 with diversions now in place and the entry slip road at Rivington Services has also been shut.
Police are advising drivers to follow the hollow black square on signs to take a diversionary route.
Highways England said maintenance crews were on the scene.
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