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Approximately 8000 lignite exploration cores, each 91 m (300 ft) deep, were used to map the gravel facies of the Upland Complex (Lafayette gravel) preserved on drainage divides in western Kentucky and Tennessee and on Crowley's Ridge in southeastern Missouri and eastern Arkansas. The Upland Complex is interpreted to be the remnant of a high-level terrace of the ancestral Mississippi-Ohio River system. The longitudinal profile of the Upland Complex and its projection on sea-level curves suggest that this alluvial deposit is early Pliocene in age (5.5–4.5 Ma). Sea level during the early Pliocene was +100 m, and the Upland Complex is interpreted to have been an ∼100-m-thick floodplain when initially deposited. Sea-level decline to −20 m at 4 Ma resulted in incision through the Pliocene floodplain, which formed the high-level terrace. Incision through the floodplain occurred in the Western and Eastern Lowlands of eastern Arkansas and their tributary valleys. The upper silt and sand facies of the terrace (∼60 m) were eroded, leaving the basal gravel-rich Upland Complex preserved on drainage divides.

The New Madrid seismic zone lies beneath the Eastern Lowlands. There has been up to 100 m of denudation above the seismic zone in the past 4 m.y., and the most recent denudation occurred in the Holocene due to the confluence of the Mississippi and Ohio Rivers stepping north to Thebes Gap, Missouri. The late Wisconsin and Holocene denudation may have perturbed the local stress field and reactivated the New Madrid seismic zone.

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