Early Tertiary Climate Change and its

Impact on Matrix Mineralogy of the

“auriferous gravels”

in the Sierra Nevada Foothills

Jim Wood

Sierra Geological Services

J. Reed Glasmann, PhD

Willamette Geological Services

Abstract“Early Tertiary auriferous gravels” is a non-descript early mining term that remains in use

on modern maps of both the USGS and the California Geological Survey (CGS).

Additional terms such as “lower gravel/upper gravel” and “channel gravel/bench gravel” all

once held important economic connotations for 19th century miners but few modern

geological studies have correctly characterized the physical and temporal nature of these

important sedimentary deposits.

Recent field investigations and analytical data collected from several locations of the

“auriferous gravels” throughout the Sierra foothills show this map unit to be a complex

sedimentary assemblage comprised of at least two geologic units possessing drastically

contrasting mineralogical and age relationships. Kaolinitic quartz-rich fluvial sandstones

dominate the “lower or channel gravels” and represent the proximal Ione Formation fluvial

system (≈50Ma). In contrast, the “upper or bench gravels” is a smectitic unit with complex

sandstone matrix mineralogy and represents a much younger fluvial system (≥ 30 Ma).

These smectitic sediments are best represented at Chalk Bluff in Nevada County. In

addition, similar kaolinite/smectite mineralogical distinctions are observed in the paleosols

buried immediately below these respective fluvial units. The mineralogical record

contained in these sedimentary and pedologic units collectively forms an important addition

to other climatic data indicating major global climate change at the end of the Eocene

period. Thus, the 19th century nomenclature spawned during the heyday of gold mining in

California should be abandoned in favor of a new classification based upon important

differences in matrix mineralogy. Such revised classification will help more clearly define

the character of these Early Tertiary sediments and their important role in understanding

the complex geological evolution of the Sierran region as well as global climate change

during this dynamic period.2

What are the “auriferous gravels”?(prevailing understanding)

• River system sediments

• Middle Eocene age

• Quartzose sand with kaolinitic matrix clay

• Proximal deposits in Sierra foothills equivalent to Ione Formation

– Coarse grained sandstone and conglomeratic sandstone

– Gold bearing

• Distal fluvial/deltaic Ione deposits in fringe zone of Sacramento and

San Joaquin Valley province

– Finer grained sandstone and claystone

– Ione and Lincoln are best known localities

– Mined for kaolinite and quartz sand for glass

3

Earliest classification scheme of “auriferous

gravels” was defined by miners

• Miners used gold content to define units:

– Highest concentrations in “channel gravels or lower

gravels”

– Lower concentrations in “bench gravels or upper

gravels”

• Early geologists adopted this mining

nomenclature

4

Early Tertiary River System

Architecture

“channel gravels”

“bench gravels”

(terrace deposits)

5

Archaic 19th Century mining terminology is still

in use today on USGS and CGS maps and in

other geologic literature

Continued usage has perpetuated various

misconceptions and ignores important

mineralogical distinctions that have

implications for understanding Sierran Tertiary

geohistory and paleoenvironments

6

History of Current Research

• Studies began in 1980’s at UNOCAL

Research Center

• Objective to gain better understanding of

kaolinitic petroleum reservoir sands

• Conducted provenance study with Ione

Formation fluvial system as a model

7

J. D. Whitney (1873)

AltaYou Bet /

Chalk Bluff

Buckeye Hill

Quaker Hill /

Scott’s Flat

Nevada City

WashingtonPrinciple study locations in

Sierra foothill areas

Iowa Hill

La Porte

40mi

8

Global Early Tertiary Climates

Were Very Humid and Warm

• Climate moisture and temperature regimen

played an important role in determining the

composition of soils and sediments

– Weathering of primary minerals

– Formation of secondary clay minerals

9

• Hydrolysis is the principle agent of chemical weathering

of minerals in soils

• Water forms a weak acid

– H2O molecule is bi-polar

– dissociates into H+ and OH– ions

• Hydrolytic intensity depends on volume of water moving

through the soil

• Higher temperature accelerates chemical reactions

Chemical Weathering

+

–

+

–

10

• Various minerals’ susceptibility to

weathering and decay depends on

strength of bonding:

– Ionic bonding more susceptible

– Covalent bonding more resistant

• Silica polymerization (structure complexity)

Chemical Weathering

11

Weathering Sequence Follows Bowen’s Reaction Series

12

Secondary clay mineral stability in soil

determined by hydrolytic intensity

• Factors determining secondary clay

mineral formation:

– Availability of cations (K, Na, Ca, Fe, Mg)

– Availability of Si determines the number of silica

tetrahedral layers

13

2:1 clays

1:1 clay

14

Early Tertiary Climate Trend

Modified from : Zachos, James, Mark Pagani, Lisa Sloan, Ellen Thomas, and Katharina Billups (2001). "Trends,

Rhythms, and Aberrations in Global Climate 65 Ma to Present". Science 292 (5517): 686–693.

doi:10.1126/science.1059412

Ione

Increasing

hydrolytic

intensity

Should expect

change in climax

clay assemblage in

soils and sediments

15

Preservation of Early to Middle

Eocene Soils

Rising channel fill

Preserved Paleosols

River system confined to channel

16

Mid-Eocene Regional Soils

• Paleo “Oxisols” preserved below Ione Formation

sediments in proximal and distal areas

• Kaolinite was the climax clay specie

• All weatherable minerals absent

• Ephemeral (transitory) clay minerals only in

weathering front

• Example: paleo Oxisol — Nevada City

17

Oxisol Clay Mineralogy

Oriented clay mineral XRD analysis (<2µm)

SaproliteWeathering front

S

I

K

K

I

18

The matrix mineralogy of fluvial

sediments that were derived from

the erosion of regional soils should

mimic the climax clay mineralogy of

the soils

19

Ione proximal sandstone matrix clay

assemblage (near Washington)

Kaolinite is dominant clay mineral

K

I

V

Gib

K

20

Distal Ione Sandstone

Kaolinite is dominant clay mineral

K

Ione

K

Lincoln

21

What happened in the last 10 meters

of Ione claystone section?

(Wood, Glasmann, and Stout 1995)

Gradual increase in smectite

content in top 10 meters

Same mineralogy in Lincoln

claystone section

~5% S

~30% S

22

What is the significance of the

sudden appearance of smectite at

the top of the Ione section?

Answer lies in the upper

sedimentary section of the

“auriferous gravels” — aka terrace

or bench deposits

23

Early Tertiary River System

Ione channel-fill

sediments (kaolinitic)

terrace bedrock paleosols

developed before

appreciable sediment

covered them

24

Soil and Sediment Sequence of

Terrace Deposits at Chalk Bluff

photo from Howard Schorn, 2012

fluvial

terrace

25

Hydraulicked

channel

gravels

Weathering Front in

Terrace Bedrock Surface

XRD shows incipient alteration of slate bedrock. Secondary minerals

—smectite, kaolinite, and illite (sericite). The latter may be inherited

from parent rock.

1

1

1

2

3

5 10 15

x10^3

5.0

10.0

15.0

20.0

Inte

nsity(C

ounts

)

JW 1302-01 <2um Mg-Glycol <2T(0)=-0.065>JW 1302-01 <2um Mg-AD <2T(0)=-0.145>

(1) Smectite Mg-Glycol, hi disorder

(2) Illite, low disorder

(3) Kaolinite, max N=50, dfd = 30

(4) Quartz, syn - SiO2

(5) Goethite - FeO(OH)

S K

I

26

Paleosols on terrace bedrock surface

.

1

2

2

2

3

4 4

5 10 15

x10^3

5.0

10.0

15.0

20.0

Inte

nsity(C

ou

nts

)

JW Saprolite <2um Mg-GlycolJW Saprolite <2um Mg-AD

Shift to smectite as the dominant clay specie (1:1 to 2:1)

Chalk Bluff

1

1

1

2

3

3

4

5 10 15

x10^3

5.0

10.0

15.0

20.0

25.0

Inte

nsity(C

ounts

)

JW-La Porte 0811-9 <2um Mg-Glycol, SCAN: 2.0/40.0/0.04/3(sec), Cu(35kV,30mA), I(max)=27651, 04/15/13 17:38 <2T(0)=-0.195>

JW-La Porte 0811-9 <2um Mg-AD, SCAN: 2.0/14.0/0.04/3(sec), Cu(35kV,30mA), I(max)=18900, 04/13/13 11:09 <2T(0)=-0.195>

La Porte

S

K

S

K

27

Paleosol buried by ash flow tuff

~30Ma

1

1

1

2

3

34

5

11

5 10 15

x10^3

5.0

10.0

15.0

Inte

nsity(C

ou

nts

)

JW-Summit 1209-1 <2um Mg-Glycol, SCAN: 2.0/40.0/0.04/3(sec), Cu(35kV,30mA), I(max)=18939, 04/15/13 16:43 <2T(0)=-0.23>JW-Summit 1209-1 <2um Mg-AD, SCAN: 2.0/14.0/0.04/3(sec), Cu(35kV,30mA), I(max)=17647, 04/13/13 11:27 <2T(0)=-0.23>

Weathered granitic knoll

entombed by ash flow

S

K

Donner Summit

28

Is this the new climax clay mineral assemblage of

regional soils after significant climate change?

.

1

2

2

2

3

4 4

5 10 15

x10^3

5.0

10.0

15.0

20.0

Inte

nsity(C

ou

nts

)

JW Saprolite <2um Mg-GlycolJW Saprolite <2um Mg-AD

Matrix clay assemblage in fluvial sediments should mimic the clay assemblage of regional soils

Chalk Bluff

1

1

1

2

3

3

4

5 10 15

x10^3

5.0

10.0

15.0

20.0

25.0

Inte

nsity(C

ounts

)

JW-La Porte 0811-9 <2um Mg-Glycol, SCAN: 2.0/40.0/0.04/3(sec), Cu(35kV,30mA), I(max)=27651, 04/15/13 17:38 <2T(0)=-0.195>

JW-La Porte 0811-9 <2um Mg-AD, SCAN: 2.0/14.0/0.04/3(sec), Cu(35kV,30mA), I(max)=18900, 04/13/13 11:09 <2T(0)=-0.195>

La Porte

S

K

S

K

1

1

1

2

3

34

5

11

5 10 15

x10^3

5.0

10.0

15.0

Inte

nsity(C

ounts

)

JW-Summit 1209-1 <2um Mg-Glycol, SCAN: 2.0/40.0/0.04/3(sec), Cu(35kV,30mA), I(max)=18939, 04/15/13 16:43 <2T(0)=-0.23>

JW-Summit 1209-1 <2um Mg-AD, SCAN: 2.0/14.0/0.04/3(sec), Cu(35kV,30mA), I(max)=17647, 04/13/13 11:27 <2T(0)=-0.23>

Donner Summit

S

K

29

Chalk Bluff terrace sediments

immediately above paleosol

1

1

1

2

33

4

56

5 10 15

x10^3

5.0

10.0

15.0

20.0

25.0

Inte

nsity(C

ou

nts

)JW 1302-03 <2um Mg-Glycol <2T(0)=-0.11>JW 1302-03 <2um Mg-AD <2T(0)=-0.11>

(1) Smectite Mg-Glycol, hi disorder

(2) Smectite Mg-hyd, disordered

(3) Illite, low disorder

(4) Kaolinite, max N=50, dfd = 30

(5) Dehydrated Halloysite (Bellpine) Mg-Sat 60C

(6) Goethite - FeO(OH)

(7) Quartz, syn - SiO2

Kaolinite is still dominant

Smectitic material eroded

from contemporary soils was

diluted by amalgamation of

kaolinitic Ione sediment in the

river channels upstream

“Matrix Clay Mineral Response Lag”

S

K Cause:

30

Initial fluvial sediments deposited

on terrace surface

kaolinitic Ione

sediments

Initial Terrace

Deposits

Smectitic

paleosols

Stream scouring of underlying

kaolinitic Ione sediments in upstream

reaches

31

Sandstone higher in the section

1

1

1

2

3

4

5 10 15

x10^3

2.0

4.0

6.0

8.0

10.0

Inte

nsity(C

ou

nts

)

JW Chalk Bluff Base Up. Ione SS <2um Mg-GlycolJW Chalk Bluff Base Up. Ione SS. <2um MgAD

Smectite content increasing

As underlying Ione

sediments become covered,

their influence on the

sediment matrix

composition diminishes

32

Terrace sandstones at yet higher

stratigraphic level

1

1

1

2

3

4

5

5 10 15

x10^3

5.0

10.0

15.0

20.0

25.0

Inte

nsity(C

ounts

)

JW 1302-04 <2um Mg-Glycol <2T(0)=-0.105>JW 1302-04 <2um Mg-AD <2T(0)=-0.105>

(1) Smectite Mg-Glycol, hi disorder

(2) Smectite Mg-hyd, disordered(3) Illite, low disorder

(4) Kaolinite, max N=50, dfd = 30

(5) Dehydrated Halloysite (Bellpine) Mg-Sat 60C

(6) Goethite - FeO(OH)

(7) Quartz, syn - SiO2

1

1

1

2

3

3

4

5 10 15

x10^3

5.0

10.0

15.0

20.0

25.0

Inte

nsity(C

ou

nts

)

JW-Buckeye 1302-6 <2um Mg-Glycol, SCAN: 2.0/40.0/0.04/3(sec), Cu(35kV,30mA), I(max)=25717, 04/15/13 15:47 <2T(0)=-0.145>JW-Buckeye 1302-6 <2um Mg-AD, SCAN: 2.0/14.0/0.04/3(sec), Cu(35kV,30mA), I(max)=19727, 04/13/13 10:50 <2T(0)=-0.145>

Chalk Bluff Buckeye Hill

Smectite/kaolinite 50/50

Quaker Hill

1

1

2

34

5

6

5 10 15

x10^3

5.0

10.0

15.0

20.0

Inte

nsity(C

ounts

)

JW B-4 62ft <2um Mg Glycol <2T(0)=-0.05>JW B-4 62ft <2um Mg Air Dry <2T(0)=-0.05>

KS K

S

S K

33

1

1

1

2

3

4

5 10 15

x10^3

2.0

4.0

6.0

8.0

10.0

12.0

14.0

Inte

nsity(C

ou

nts

)

JW-2 Chalk Bluff Tuff SS, upper part of seciton <2um Mg-Glycol

Sandstones near the top

of the Chalk Bluff section

Smectite (2:1 clay) dominant

Chalk Bluff AltaQuaker Hill

1

1

1

2

3

4

5 10 15

x10^3

5.0

10.0

15.0

20.0

25.0

Inte

nsity(C

ou

nts

)

JW-1 Alta Firehouse Rd SS above Gx <2um Mg-Glycol <2T(0)=-0.09>JW-1 Alta Firehouse Rd SS above gx Up. Ione <2um MgAD <2T(0)=-0.09>

1

1

2

4

5

5 10

x10^3

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

Inte

nsity(C

ou

nts

)

JW B-1 76ft <2um Mg Glycol <2T(0)=0.12>JW B-1 76ft <2um Mg Air Dry <2T(0)=0.12>

S S

K

S

KK

34

Smectitic Sediments in Distal Areas

1

1

1

2

3

45

5 10 15

x10^3

5.0

10.0

15.0

WK Lot3 >200mesh <2um Mg-Glycol <2T(0)=-0.065>WK Lot3 >200mesh <2um MgAD <2T(0)=-0.065>

Sacramento Co IoneLincoln

Sediment Matrix Clay Assemblage Mimics Regional Soil Mineralogy

33

Do we see same matrix clay trend in other regions?

Mt Soledad ss

K

Upper Mt Soledad ss Ss unit above

K

K

S

S

Mt Soledad sandstone (50Ma) at Torrey Pines beach, San Diego36

Rhyolitic Tuffs Interbedded in the Chalk Bluff

Section Confines the Upper Age

Other examples: Alta, Chalk Bluff, Nevada City, La Porte, Lincoln

“Tuff of Axehandle Canyon” ~31.5Ma

(40Ar / 39Ar : Chris Henry)

Iowa Hill

Quartzose smectitic

sediments

37

Rhyolitic tuff at the top of the Chalk Bluff

sedimentary section

Rhyolitic tuff pebbles

and boulders

38

Olig/ Mio contact

Conclusions

• The Middle Eocene to Early Oligocene transition was

a time of drastic global climate change

• The Sierran Early Tertiary sediments and the fossil

soils they bury collectively form a mineralogical

record of decreasing hydrolytic intensity associated

with such a change in the hydrologic regimen

39

Middle Eocene Soils and Sediments

Ione kaolinitic channel-

fill sediments

Kaolinitic paleo-

Oxisols

Paleosols below the Ione

Formation are Oxisols with

kaolinite being the dominant

and climax clay mineral

Ione Formation sediments

in proximal and distal areas

reflect this regional kaolinitic

soil mineralogy

40

Terrace Bedrock Soils

Paleosols developed on the terrace bedrock

surfaces and the regional landscape show a

striking transition from kaolinite to smectite (1:1

clay to 2:1 clay) indicative of lower hydrolytic

intensity

41

Transition Terrace Sediments

The clay mineralogy of the sediments

above the bedrock terrace show a

gradual transition to greater smectite

content higher in the section

40

Upper Terrace SedimentsSediments in the upper section of

the terrace deposits as well as

distal areas mimic the regional

soil mineralogy dominated by

smectite with minor kaolinite

Interbedded tuffs at the top of the

Chalk Bluff section date to 33Ma to

30Ma

43

Age of Chalk Bluff sedimentary interval?

Source : Zachos, James, Mark Pagani, Lisa Sloan, Ellen Thomas, and Katharina Billups (2001). "Trends, Rhythms, and

Aberrations in Global Climate 65 Ma to Present". Science 292 (5517): 686–693. doi:10.1126/science.1059412

?

K

S

Chalk Bluff Interval

Decreasing

hydrolytic

intensity

Ione

44

Need for New Nomenclature

• The sequence of smectitic terrace

sediments known as the “bench gravels”

or “upper gravels” and interbedded tuff

beds at the top of the Chalk Bluff section

was a sedimentary system that operated

during a later time of distinctly different

environmental conditions than those of the

Middle Eocene.

45

Thanks to:

• Chris Henry (UNR) for 40Ar/39Ar dates

• The Brady family allowing access to

their properties at Chalk Bluff

• Kathy Morgan allowing access to her

mine property at Iowa Hill

46