Here's why:
* Time Gap: The Jurassic Period (201-145 million years ago) is separated from the Pennsylvanian Period (323-299 million years ago) by a significant amount of geologic time (approximately 150-220 million years).
* Erosion and Uplift: During this time, substantial erosion and uplift likely occurred in the region, leading to the removal of a large amount of rock that was deposited during the Pennsylvanian. This means that there is a significant time gap (an unconformity) between the coal deposits and the dinosaur fossils.
* Transgression and Regression: The Jurassic Period saw dramatic shifts in sea level. The deposition of coal in the Pennsylvanian suggests a swampy, low-lying environment. The presence of dinosaur fossils in the Jurassic indicates a more terrestrial environment. These changes in depositional environments over time also contribute to the discontinuity.
In summary: The presence of Jurassic fossils on top of Pennsylvanian coal deposits is a clear indication of a substantial time gap and an interruption in deposition, not a continuous, uninterrupted sequence of events.
