What has drilling on the lower Delmarva Peninsula revealed about its geological history?
Drilling on the lower Delmarva Peninsula has revealed that a massive asteroid or comet impact 35 million years ago dramatically altered the region's geological makeup. Long before the Chesapeake Bay existed, this event shattered and displaced the ancient rock layers, creating a deep crater that still influences the land and water today.
The big impact and its after-effects
- A buried, deep crater: A huge impact crater, roughly 53 miles wide and a mile deep, is buried under the southern part of the Chesapeake Bay and the lower Delmarva Peninsula. It was discovered through a combination of drilling and seismic surveys, which allowed geologists to map its shape and features.
- Aquifers were destroyed: The impact obliterated ancient aquifers (underground layers of water-bearing rock), creating a massive "breccia lens"—a jumbled mix of rock, sediment, and fractured material. The aquifers today are freshwater on top, but the deeper ones are still mixed with ancient, salty seawater trapped in the crater.
- An inland saltwater pocket: This deep brine has remained trapped within the crater's structure, and its unusually high salinity was a long-standing mystery to scientists before the crater's discovery.
- The origin of the Chesapeake Bay: The impact created a long-lasting depression in the landscape. As sea levels rose and fell over millions of years, rivers in the region converged and were eventually funneled into this low-lying area, ultimately forming the present-day Chesapeake Bay.
Layers of sediment tell a story
- Layers from old river deltas and seas: The region has been covered and uncovered by the ocean multiple times over millions of years. The deep drill cores show layers of sediment deposited by rivers, including a large river delta system that existed in the Late Miocene epoch, and marine deposits from ancient seas.
- "Exotic" plant fossils: Microscopic fossils of plant material found in the sediments are not native to the area, showing that the region's environment has changed significantly over time.
- A record of changing climates: Younger layers of sediment show fluctuations between periods of warm and cold temperatures. For example, some layers show a warm-temperate environment, while younger ones indicate a cooler climate.
The peninsula continues to change
- Groundwater vulnerability: The fractured ground and persistent saltwater deep in the crater mean that the area's fresh groundwater is more susceptible to contamination.
- The peninsula is sinking: The entire Delmarva Peninsula is slowly sinking, a long-term effect of the impact crater's influence. This process shapes the landscape and impacts the region's hydrology, even today.
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