Insolation forcing of sub-lacustrine debris flows – Could monsoon intensification have played a role? Eocene lacustrine Green River Formation, Piceance Creek Basin, Colorado

Abstract

Mudstone breccias deposited by debris flows in the lacustrine Green River Formation of the Piceance Creek Basin, Colorado, may not represent random and unpredictable events as previously thought. Spectral analysis of mudstone breccia beds in the Wolfridge Minerals John Savage #24-1 basin center core reveals a record of short eccentricity (94.58 kyr), obliquity (50.78 kyr and 39.26 kyr), and precession (21.85 kyr and 18.56 kyr). Of particular interest to this investigation are the records of eccentricity and precession, together which indicate that insolation forcing of monsoons occurred every ~18–21 kyr, and that every ~94 kyr, insolation maxima exceeded a threshold of 528 W/m2, resulting in more significant monsoon amplification. Strengthened monsoon activity would have led to an increase in precipitation and runoff delivered to the lake. This increase in inflow resulted in lake-level rise which, in turn, decreased basin slope stability and triggered periodic sub-lacustrine debris flows that deposited the mudstone breccias.