Taken from (but slightly adapted): http://www.oldearth.org/flood.htm
... creation science proponents are quick to use dinosaur graveyards as evidence of Noah’s Flood. They claim the dinosaurs herded together, and then were quickly buried. However, this explanation is not feasible.
The dinosaur graveyards referred to are mostly in North America, in sediments in Utah, Wyoming, Colorado, Montana, and Canada. However, looking at the positioning of the rock layers, there are thousands of feet of sediment below these layers that the young earth theorists claim were deposited by the Flood.
To make this more understandable, let’s look at the Grand Canyon. Steven Austin, in his book Grand Canyon: Monument to Catastrophe, claims the Canyon rocks represent those which were deposited during the rising waters phase of the Flood (Figure 4.1). The “Late Flood”, or receding water rock deposits, are the Mesozoic sediments.
It is interesting to note that all the dinosaur fossils, including the mass graves, are Mesozoic in age. This means that all the dinosaurs died in the receding water phase of the flood. However, it is clear from Genesis 7:21-23, that all life was killed during the first 40 days of the Flood. Some young-earth theorists will argue that the bodies floated around, and eventually sank, based on various factors as body size, density, and so forth. However, this cannot be true, because the dinosaur footprints all exist in the same Mesozoic rock layers, as do all the dinosaur coprolites (fossilized dinosaur poop), and fossilized dinosaur eggs. Clearly, the dinosaurs were alive and well, after the declaration in Genesis 7:21-23 that all living things were killed during the first forty days of the flood.