Actually, it isn't very steady. If I could find a better seasat photo you can see just how uneven the floor of the oceans and seas are. You can see just how many catastrophic events have occured and no doubt stirred up this stuff at various times, certainly enough to make it impossible to determine what was were and when.
There are many unaswered questions which make it impossible to determine the age of anything that may be on the floor of the Pacific and Atlantic. For example, the grand Canyon is assumed to have been formed by millions of years of errosion. If that's what formed it, wouldn’t you expect to find a gigantic river delta where the Colorado River enters the Gulf of California? It’s not there. Where did 800 cubic miles of dirt go?
If it's buried under all the magna that sits on top of the actual Pacific floor, which is only assumed to be basalt like the Atlantic ocean floor, (nobody has been able to drill a hole deep enough to find out) it stands to reason anything on the pacific floor would only be as old as the dirt that has piled up from the grand canyon since that event.
Using plate tetonic theory, itself a theory that is in no way free of it's own problems creates even more questions than it answers. Plate tectonics claims a trench forms as a plate dives down into the mantle, a process called subduction. The fact that there is NO distortion as you point out of the horizontal sediment layer in these trenches shows that this can't be what's happening.
If a 30 mile thick plate was sliding into the mantle even only a couple of miles, the friction would be far greater than the rocks strength. We would see much buckling, distortion, breaking and crushing. Yet, there is no horizontal distortion.
Plate tetonic theory suggests earthquakes form where these plates slide under each other , so there would definately be a lot of activty in this area disturbing the sediment greatly. Or, tetonic theory is wrong.
Shallow earthquakes displace the ground horizontally along a fault, and would also cause heat. Since these sediments aren't disturbed or destroyed by heat, these trenches can't be caused a fault line either.
To establish a plausible theory about climate history based on sediments which are impossible to be certain of their age and reliability is a waste of time. certainly not "science" governments enviroment activists should be using to claim the sky is falling, and creating any global warming policy on.