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Joint Meeting Pacific Section, AAPG & Cordilleran Section GSA April 29–May 1, 2005, San José, California

Preliminary Research on the Relative Age Estimates of Shorelines Associated with Pluvial Lake Madeline, Lassen County, Northeastern California

Ronna Bowers and R. M. Burke
Department of Geology, Humboldt State Univ, Arcata, CA 95521, [email protected]

Shoreline remnants record the existence of pluvial Lake Madeline, an internally drained lake system within the southeastern Modoc Plateau region of California. At maximum lake levels this system covered ~777 km2, connecting what is now Grasshopper Valley, Dry Valley, and the Madeline Plains. Preliminary soil data and field mapping were used to establish relative age estimates and correlate shoreline surfaces. Evidence of 16 soil profiles indicates that a suite of approximately eight shoreline surfaces represent a minimum of two different ages of lake cycles. Disparities among fundamental soil properties define the relative age break between surface altitudes 1654 m and 1651 m.

Preservation of shorelines at and above 1654 m is limited. These landforms are discontinuous and dissected; surfaces have relatively few, subrounded-rounded lacustrine beach gravels. The 1654 m shoreline soil is a 161 cm profile of A/Bw/Bt1/Bt2/Bt3/2Bkm1/2Bkm2/Coxk that has a strongly cemented, pedogenic carbonate matrix containing well-rounded, gravels and small cobbles. Thin, 2 mm, discontinuous carbonate laminations in the 2Bkm1 horizon are indicative of Stage IV carbonate development. Eolian sediments cap the carbonate-rich horizons, showing strong, medium-coarse, prismatic structure in a fine-grained SiL–SiC texture.

Shorelines at or below 1651 m are continuous and well-preserved, having many rounded lacustrine beach gravels and cobbles of various lithologies on the surfaces. Soils developed on these shorelines have weak-moderate structure without pedogenic carbonate development. In particular, the 1646 m shoreline soil is a 64 cm profile of Av/Bw1/Bw2/2Bw overlying a bedrock platform. Associated with the younger set of shorelines are basin lake sediments containing the Trego Hot Springs tephra (~23ka B.P.; Foit, 1995).

These soils data do not provide a continuum of ages, but rather two groups of ages. Preliminary age assignments and tephra correlation indicate that the younger set of soils is MIS 2 equivalent and the older set of soils is MIS 6 equivalent or older. Results demonstrate how soil development decreases with decreasing altitude, suggesting shorelines here are recessional features and that subsequent lake stands of pluvial Lake Madeline were lower than that obtained during deposition of the 1654 m shoreline.

Posted with permission of The Geological Society of America; abstract also online (http://gsa.confex.com/gsa/2005CD/finalprogram/abstract_85834.htm). © Copyright 2005 The Geological Society of America (GSA).