Radio-Isotope Dating: Errors

Manson Crater in Iowa is dated at 75 MYa. Chixculub Crater in the Gulf of Mexico, the extinction event of the dinosaur age - at 65 MYa. 

It is possible that the measurements for age under water are not standardized to the ones established in land-based laboratories.

Like light bending at the air-water interface, radioactive decay may be slowed under water.

One way to test this would be to subject various radioactive materials to a variety of depths of water (measured to the edges of a sphere) and measure for changes in the rate of decay.

It is entirely possible that the two craters actually arose simultaneously - this is congruent with the pattern of fragmentation seen in Shoemaker-Levy 9 (21 fragments).  This pattern suggests multiple impacts and also predicts other craters from this group yet to be named as such.

In addition, the age given for the ocean floor - 200 Million years - might be low given the slowing of decay underwater.