SUMMARY OF SEDIMENTOLOGIC INVESTIGATIONSIN MEDICINE LAKE AND BLANCHE LAKE,NORTHERN CALIFORNIAR. Lawrence Phillips, John A Barron, and Scott W. Starratt
Ten cores, ranging from 18 to 70 cm in length, were obtained in September 1999 in northern California, nine from Medicine Lake and one from Blanche Lake (Figure C3, Table 1). The cores were taken to assess whether Medicine Lake may contain a high-resolution record of the Holocene climate history of the region. The climate history would be inferred through studies of microplankton, pollen and lake & sediment geochemistry.

The most obvious climate cycle that we expect to be present in Medicine Lake sediments is that of lake level. Because Medicine Lake is spring-fed, lake-level variations should correlate with regional precipitation. For example, according to eyewitness accounts, the lake was much smaller during a well-documented period of extended drought during the 1930s. Given the lake's bathymetry, with extensive areas shallower than 10 feet (~ 3m), extended drought would drastically alter the lake area. Preliminary interpretation of the sediments from the shallow-water cores (Cores 2-8) reveals clearly defined cycles of lake level that we hope to date and relate to published climate records. Though the record contained in the shallow-water sediments undoubtedly is interrupted by periods of subaerial exposure or erosion, sediments from the deeper part of the lake (Core 9) are likely to contain an uninterrupted record of lake level. Diatoms and geochemistry have proven to be excellent monitors of lake level, so they will be studied extensively in the deep cores. 

This report summarizes the coring methods, lithostratigraphy, texture, and depositional environments identified in the core strata from this inital study.

Summary

Data from the shallow lake cores show that an initial lagoonal/marsh environment was drowned, allowing coarse-grain lake sediments to be deposited. Subsequently, there was a drop in lake level and subaerial soils were developed on the lacustrine strata. A rise of lake level in 1940's(?) again flooded the terrestrial soils and resulted in deposition of the thin modern lake sediments.
- Sampling   and Analytical Methods 
- Stratigraphy   Summary Diagram  (Figure C1)   
- Key   to Sedimentary Structures  (Figure C2)   
- Map   of Sample Locations  (Figure C3)   
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Table 1. Information on core locations and characteristics.
CoreDepth(cm)Length(cm)NorthLatiitudeWest LongitudeCoretypeArea
Strat. Column  

Textures  
 107041° 35.3'121° 36.2'HammerBarrier Island north side Medicine LakeStrat.       ColumnTexture       Column 21006941° 35.6'  121° 35.6'HammerNorth side Medicine LakeStrat.       ColumnTexture       Column 31303141° 35.2'  121° 36.2'HammerNorth side Medicine LakeStrat.       ColumnTexture       Column 45062.541° 34.6'  121° 35.6'HammerLagoon north side Medicine LakeStrat.       ColumnTexture       Column 51103141° 34.7'  121° 36.5'HammerSouth side Medicine LakeStrat.       ColumnTexture       Column 610018.541° 35.6'  121° 35.6'BoxcoreNorth side Medicine LakeStrat.       ColumnTexture       Column 71001841° 35.6'  121° 35.6'BoxcoreNorth side Medicine LakeStrat.       ColumnTexture       Column 83049.541° 33.4'  121° 33.2'HammerNorth side Blanche LakeStrat.       ColumnTexture       Column 937002741° 34.9'  121° 35.4'GravityEast central Medicine LakeStrat.       ColumnTexture       Column
Depositional environments of Medicine Lake

Six cores were obtained in the northern part of the Medicine Lake, where a narrow coarse-grained barrier island has built to the east into the lake and now separates a broad, shallow, grass-covered lagoon from the open lake (Figure 2). One core (core 5) was taken along the southern margin of the lake, and the gravity core (core 9) was taken in the deeper, eastern part of the lake.

We identify at least six distinct lithostratigraphic sequences in the lake strata that include the following depositional environments: 1.) Modern shallow lake, 2.) Deep lake, 3.) Barrier island, 4.) Lagoonal and barrier island overwash, 5.) Terrestrial strata, and 6.) Lacustrine/marsh to lagoonal sediments.
- Modern shallow lake sediments   
- Modern sediments within the shallow part of the lake are now being   reworked. They overlie consolidated sediments with an erosional contact.   Discontinuous patches of grass 2 to 3 cm in height may cover part of the   shallow lake where actively migrating ripples are not present. The modern lake   sediments form a distinctive unit ranging from 2 to 14 cm thick.   Coarse-grained sand and gravel make up to 100 percent of the sediment. Silt   and clay are generally lacking or constitute less than 3 percent of the   sediment in this upper stratigraphic interval. The sand fraction is composed   of a mixture of tephra clasts, consisting of red, black, tan to cream colored   vesicular shards and clear bubble wall shards.   
- Deep lake sediments   
- The soft dark brown deep lake sediments contain the highest percentage of   silt and clay (up to 96 percent) with the sand fraction ranging from 3.6 to   32.7 percent (core 9). Rare gravel clasts (possibly ice-rafted) are also found   in the soft mud. Most of the sand-size fraction consists of biologic   components consisting of diatoms, fern and conifer spore cases, seed pods,   plant material, and insect parts.   
- Barrier island   
- The barrier island strata consist of 70 cm of repeated graded beds of   gravel and sand (core 1). The gravel fraction ranges from 0.4 to 60 percent   while the sand fraction ranges from 42.2 to 99.9 percent. The sand and gravel   fractions are composed of a mixture of tephra clast compositions. The strata   represent repeated winnowing and deposition of coarse-grain sediments by waves   and wave runup on the narrow barrier island beach.   
- Lagoonal and barrier island overwash     
- The lagoonal strata, 30 cm thick, comprise two   distinct depositional elements, a basal laminated gray and brown sandy-silty   clay lacking gravel-sized-clasts overlain abruptly by coarse-grained sandy   gravels. The silt and clay fraction makes up to 48 percent of the sediment in   the basal beds changing up 10 to 16 percent in the overwash strata. The sand   (up to 76 percent) and gravel (up to 20 percent) form a mixture of tephra   grains and clast types in the upper depositional unit. The strata represent   quiet water deposition for the basal strata with barrier-island overwash   introducing the gravelly sands into the lagoon.   
- Terrestrial strata   
- The hard, overconsolidated, red to orangish brown sandy mud, which   underlies the modern lake and lagoonal sediments, contains iron concretions,   silt clasts, and black to brown roots to 10 cm length forming a distinctive   unit ranging in thickness from 16 to 22 cm. An abrupt increase in the   <0.062 mm size fraction, from 38 to 78 percent, characterizes this   sequence. The sand fraction ranges from 22 to 56 percent. Gravel-sized tephra   clasts are usually lacking. Most of the sand-size shards are altered. Apparent   exposure of originally lacustrine and lagoonal/marsh sediments has resulted in   soil development (high silt and clay fraction) and alteration of the sediment.   The sequence was deposited in a terrestrial environment.   
- Lacustrine/marsh to lagoonal   sediments   
- The stratigraphically lowest depositional sequence found in cores 2, 3,   and 4 contains two distinct units, a lower marsh/lagoonal sequence up to 26 cm   thick overlain by an upper thin lacustrine deposit 4 to 13 cm thick. The   lacustrine strata consist of "clean" sand (97 to 100 percent) or gravelly sand   (4 to 16 percent gravel, 52 to 75 percent sand) composed of a mixture of   tephra compositions. The laminated lagoonal/marsh sediments contain 48 to 71   percent silt and clay and 28 to 51 percent sand indicating deposition in a   quiet-water environment.   
- Blanche Lake   
- Blanche Lake contains a similar depositional history as recorded in the   strata in Medicine Lake with a thin, 6 cm thick, sandy gravel representing the   modern lake sediments overlying gravelly, sandy mud from a terrestrial   environment with soil development. The gravel clasts are scattered throughout   the strata with clasts up to 6.2 cm in length.   
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