Quaternary Geologic Map of the North-Central Part 

of the Salinas River Valley and Arroyo Seco, Monterey 

County, California

Pamphlet to accompany

Scientific Investigations Map 3260

U.S. Department of the Interior

U.S. Geological Survey
Cover: View looking downstream at a well-preserved sequence of late Pleistocene and inset Holocene strath terraces. Where 

Arroyo Seco has eroded Tertiary marine sediments, primarily Monterey shale, a broad valley has formed. Each terrace records a 

floodplain eroded into the bedrock and is capped by rounded gravel derived from the more resistant basement rocks upstream. 

Photograph by Tony Garcia, California Polytechnic University, San Luis Obispo, 2012.
Quaternary Geologic Map of the 

North-Central Part of the Salinas River 

Valley and Arroyo Seco, Monterey County, 

California

By Emily M. Taylor and Donald S. Sweetkind

Pamphlet to accompany

Scientific Investigations Map 3260

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

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Suggested citation:

Taylor, E.M., and Sweetkind, D.S., 2014, Quaternary geologic map of the north-central part of the Salinas River 

Valley and Arroyo Seco, Monterey County, California: U.S. Geological Survey Scientific Investigations Map 3260, 

13 p., 2 sheets, scales 1:24,000 and 1:50,000, http://dx.doi.org/10.3133/sim3260.

ISSN 2329-132X (online)
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Geologic Setting.............................................................................................................................................3

Description of Strath Terraces ....................................................................................................................5

Methods...........................................................................................................................................................6

Discussion and Summary.............................................................................................................................9

Acknowledgments.........................................................................................................................................9

Description of Map Units............................................................................................................................10

References Cited..........................................................................................................................................12

Figures

1. Location map .................................................................................................................................2

2. Photographs showing stream-valley morphology of Arroyo Seco ......................................3

3. Location of Arroyo Seco inset map of Quaternary-age strath terraces..............................4

4. Photographs showing lag gravels and strath terraces..........................................................5

5. Buried terrace gravel exposed in road cut in Arroyo Seco...................................................6

6. Unique soils are developed on the terrace surfaces .............................................................7

7. Stream gradient profile of terrace surfaces along Arroyo Seco..........................................8

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

yard (yd) 0.9144 meter (m)

Quaternary Geologic Map of the North-Central 

Part of the Salinas River Valley and Arroyo Seco, 

Monterey County, California

By Emily M. Taylor and Donald S. Sweetkind

Bay. Based on data from oil and gas exploration drill holes 

and deep water wells, the Salinas River valley is filled with 

as much as 1,800 m of Tertiary and Quaternary marine and 

terrestrial sediments (Durham, 1974). Preserved stream-terrace 

deposits within and near sedimentary basins are a record of 

erosional history that provides a context for downstream basin 

aggradation, basin-margin fault-related uplift, and paleoclimate events. Understanding the erosion of sediment from 

Arroyo Seco contributes to understanding the evolution of 

sedimentation in the northern part of the Salinas River valley. 

Arroyo Seco erosional strath terraces mark stable periods 

between base-level changes driven by tectonics or sea-level 

fluctuations, an indirect result of climate change. Seven of the 

terraces have been dated by feldspar infrared stimulated luminescence (IRSL) or quartz optically stimulated luminescence 

(OSL) and range in age from 1 ka to >120 ka. We combined 

terrain analysis of a digital elevation model with traditional 

geologic field methods, and relative age and dating techniques 

to develop a Quaternary geologic map to help interpret the 

long-term erosional history of Arroyo Seco drainage.

The mapped area covers six 7.5-minute U.S. Geological 

Survey (USGS) quadrangles or an area of about 33 by 28 km. 

Previous detailed bedrock mapping was done by Durham 

(1963, 1970), Page (1998) and by Dibblee and Minch 

(2006a–e; 2007). Rosenberg and Clark (2009) updated the 

mapping of the Rinconada and Reliz faults. Quaternary geologic mapping of Arroyo Seco and the northern Salinas River 

valley areas was done as part of graduate studies (Snetsinger, 

1962; Dohrenwend, 1974, 1979; Tinsley, 1975; Tinsley and 

Dohrenwend, 1979). Our new mapping provides a detailed 

stratigraphic history of incision and deposition from the early 

Pleistocene to the present.

Infiltration from the Salinas River and its tributaries is 

the source of irrigation water in this productive agricultural 

region. Irrigation water is pumped from the shallow aquifers 

at depths of about 55 and 120 m (Durbin and others, 1978); 

however, these have become intruded by seawater, especially 

between Castroville and Salinas (north of the map area). In 

those areas, groundwater is mainly pumped from the “deep” 

aquifer, which is approximately 275–400 m deep. In contrast, 

Abstract

Arroyo Seco, a perennial drainage in the central Coast 

Range of California, records a sequence of strath terraces. 

These terraces preserve an erosional and depositional history, 

controlled by both climate change and regional tectonics. 

These deposits have been mapped and correlated on the basis 

of field investigations, digital terrain analysis, stream gradient profiles, evaluation of published regional soil maps, and 

satellite imagery. Seven of the strath terraces and associated 

alluvial fans have been dated by optically stimulated luminescence (OSL) or infrared stimulated luminescence (IRSL). 

The OSL and IRSL dates on seven of the strath terraces and 

associated alluvial fans in Arroyo Seco are approximately 

>120 ka, >65 ka, 51–46 ka, 36–35 ka, 9 ka, and 2–1 ka. These 

dates generally fall within the range of ages reported from 

many well-dated marine terraces on the California coast that 

are formed during sea-level high stands. Tectonic movements, 

consistently upward, result in a constantly and slowly emerging coast line, however, the regional effects of climate change 

and resulting eustatic sea-level rises are interpreted as the driving mechanism for erosion and aggradation in Arroyo Seco.

Introduction

Arroyo Seco, a perennial drainage in the Santa Lucia 

Range of the central Coast Range of California (fig. 1) 

adjacent to the Salinas River valley, records in its deposits 

a sequence of erosional strath terraces. Arroyo Seco cuts

a narrow canyon in resistant bedrock in the uplands of the 

Santa Lucia Range, but opens eastward to a broad, meandering 

16-kilometer- (km-) long valley where the drainage flows over 

more easily erodible sedimentary rocks. Arroyo Seco declines 

in elevation from about 250 meters (m) at the canyon mouth to 

50 m, where it flows into the Salinas River valley. Arroyo Seco 

is a perennial stream and one of the largest tributary drainages 

of the Salinas River. The Salinas River valley is drained by 

the 275-km-long Salinas River, as measured from its headwaters in San Luis Obispo County northwards to Monterey 
2 Quaternary Geologic Map of the North-Central Part of the Salinas River Valley and Arroyo Seco, California

Figure 1. Location Map. Arroyo Seco is located in Monterey County in coastal, central California. Arroyo Seco, in the Sierra de Salinas,

is bound on the east by the Salinas River valley.

101

EXPLANATION

Fault

Area of Map A

Area of

Map B

CALIFORNIA

NEVADA

Monterey

Bay

Rive

T

r

ularcitos

San Antonio 

Reservoir

Nacimiento 

Reservoir

PACIFIC

OCEAN

Salinas River

Arroyo Seco

Carmel

River Salinas

River

King City

Salinas

Castroville

Greenfield

Paso Robles

Santa Margarita

MONTEREY 

COUNTY

SAN LUIS OBISPO 

COUNTY

Santa Lucia Range

Gabilan Range

Salinas River

 Valley

Sierra

de

Salinas

Carmel Valley

SAN GREGORIO

FAULT

ZONE

TULARCITOS FAULT

SAN

FAULT

ANDREAS

ZONE

RINCONADA

FAULT

ZONE

ZONE

RELIZ

FAULT

121°00' 120°30'

36°00'

35°30'

37°00'

36°30'

122°00' 121°30'

0

0

10

10

20 MILES

20 KILOMETERS

B

A
Geologic Setting 3

near Arroyo Seco and King City areas, some wells pump 

from depths of less than 15 meters. The shallower depth has 

to do with these wells commonly tapping underflow from 

the Salinas River where groundwater is kept shallow by 

controlled releases from San Antonio and Nacimiento dams 

(L. Rosenberg, USGS, written commun., 2012). The deepest part of the Salinas basin is near the mouth of Arroyo 

Seco, where depth to pre-Tertiary basement is about 1,800 m 

(Durham, 1974; Graham, 1978). The saturated thickness in 

this area is about 450–600 m, and the aquifer materials consist 

of Miocene to Pliocene sandstones within the Pancho Rico 

Formation, the Pliocene to middle Pleistocene Paso Robles 

Formation, and unconsolidated alluvium.

Geologic Setting

The Santa Lucia Range is part of the Salinian block—a 

structural block underlain by granitic and granitic-gneiss basement rocks—that lies west of the San Andreas fault system, 

in contrast to adjacent structural blocks that are characterized 

by basement rocks consisting of Franciscan Complex, Jurassic 

ophiolite, and upper Mesozoic forearc deposits (Reed and 

Hollister, 1936; Dibblee, 1973; Graham, 1978). Cretaceous 

and older crystalline granitic rock and associated metasedimentary rocks are here designated as }|ub, Paleozoic to 

Mesozoic undifferentiated basement (fig. 2). A sequence of 

Paleocene–Miocene marine sedimentary rocks, including the 

Monterey Formation, unconformably overlies the crystalline bedrock (Durham, 1963, 1970, 1974; Seiders and others, 

1983). This sequence is here referred to as Tertiary marine 

sediments (Tms) (fig. 15, sheet 2). Successively overlying 

the Tms are the Miocene to Pliocene marine sediments of the 

Pancho Rico Formation (Tpr), a Pliocene nearshore regressive 

sandstone informally named the sandstone of San Ardo (Tsa) 

(Dibblee and Minch, 2006b, c; 2007), and Pliocene to middle 

Pleistocene nonmarine alluvial sediments of the Paso Robles 

Formation (QTpr) (Durham, 1970). Quaternary units are 

subdivided into alluvial deposits, fluvial and alluvial deposits, 

fluvial deposits, and minor eolian deposits. Mapped units are 

distinguished on the basis of stratigraphic order, elevation, and 

soil development (Soil Survey Staff, 2010) primarily based on 

the accumulation of secondary clay.

Figure 2. Photographs showing stream-valley morphology of Arroyo Seco. A, narrow canyon formed in the resistant basement rocks; 

B, terraces are preserved in the resistant basement rocks as narrow treads, typically less than 5 m wide. Dashed yellow line is top of 

bedrock and dashed white line indicates the terrace surface.

A

Terrace gravel

Undifferentiated

basement rocks

B
4 Quaternary Geologic Map of the North-Central Part of the Salinas River Valley and Arroyo Seco, California

Arroyo Seco cut a narrow canyon in bedrock through the 

Santa Lucia Range (fig. 2A) that opens to a broad 16-km-long 

valley (fig. 15, sheet 2). The narrow canyon is a result of erosion 

in relatively resistant basement rocks, primarily granitic and 

metasedimentary rocks that are Cretaceous and older in age. 

Arroyo Seco opens to a broad meandering drainage at the abrupt 

contact of the basement rock with the more easily erodible 

Paleocene- to Miocene-age Tertiary marine sediments (fig. 2B). 

Arroyo Seco declines in elevation from about 250 m at the canyon mouth to 50 m, where it flows into the Salinas River valley.

Two major range-front faults intersect south of the 

confluence of Arroyo Seco and the Salinas River valley, near 

Monroe Canyon (figs. 1 and 3). The Reliz fault zone, a highangle reverse fault, trends northward, bounds the east side of 

the Sierra de Salinas, and extends to Monterey Bay (Durham, 

1970; Rosenberg and Clark, 2009). Where the fault intersects 

Arroyo Seco, there is no evidence for Quaternary offset (Durham, 1970; Rosenberg and Clark, 2009; fig. 13, sheet 2). North 

of Arroyo Seco, the Sierra de Salinas are partially flanked 

by older, undeformed (or scarcely deformed) fan deposits 

(Tinsley, 1975; fig. 2, sheet 1). Southward from Monroe 

Canyon, the Rinconada section, or “segment” as delineated 

by Dibblee (2006c), of the Rinconada fault extends to the 

Rinconada Mine near Santa Margarita (Rosenberg and Clark, 

2009, p. 5). The Rinconada fault is a right-lateral fault, similar 

to the San Andreas Fault 25 km east, with offset as young as 

Holocene (Rosenberg and Clark, 2009). Where Monroe Creek 

and Thompson Canyon drainages intersect the valley floor, the 

stream drainages are offset to the north, indicating right-lateral 

movement on the Rinconada fault. The drainage offset has 

caused incision of alluvial fans that are late Pleistocene age, 

thus supporting regional evidence for Quaternary movement. 

Figure 3. Location of Arroyo Seco inset map of Quaternary-age strath terraces. These terraces were eroded into 

Tertiary marine sediments in Arroyo Seco and the resulting sediments were deposited in the Salinas River valley. 

The Salinas River also provides a source for fine-grained sediments on the valley floor producing terrace and 

floodplain deposits at and adjacent to the river.

101

121°30' 121°20' 121°10'

36°10'

36°20'

Greenfield

King

City

Area of

inset map

Monroe

Canyon

Canyon

Reliz Canyon

Canyon

Thompson

Pine

Chalo ne Creek

Salinas River

Paloma Creek

P iney Creek

Arroyo

Seco

0

0

5 MILES

5 KILOMETERS

EXPLANATION

Fault

RINCONADA

FAULT

ZONE

RELIZ FAULT

ZONE
Description of Strath Terraces 5

Differences in the sense of displacement and amount of slip 

suggest that the Reliz and Rinconada faults are not a single 

continuous fault (Ross, 1984; Rosenberg and Clark, 2009). 

At least some of the strike-slip motion along the Rinconada 

fault zone may be transferred northwestward to the Tularcitos 

fault in the upper Carmel Valley (Rosenberg and Clark, 2009). 

Slip may be transferred along a complex assemblage of faults 

mapped in bedrock in the northern part of the map (Dunham, 

1970); it was beyond the scope of this study to attempt to 

refine the magnitude and sense of offset on these faults.

Description of Strath Terraces

Strath terraces formed on the thick sequence of the 

Tertiary marine sediments preserve the incision history of 

Arroyo Seco. Older strath-terrace deposits are preserved as 

much as about 550 m above the modern drainage whereas 

younger deposits are from 150 m to <1 m above the modern 

drainage (fig. 9, sheet 1). These terraces are capped by coarse, 

rounded granitic and metasedimentary cobbles and boulders exposed at the surface (figs. 4A and 4B; fig. 7, sheet 1). 

The rounded stones are derived from the upstream resistant 

basement rocks. These gravelly, fluvial deposits are typically 

less than 2-m thick and lie unconformably on the Tertiary 

marine sediments. The contact between the fluvial deposits 

and the Tertiary marine sediments is clear and abrupt. Soils 

are developed on the fluvial deposits. Road cuts and natural 

exposures in Arroyo Seco also expose side-stream alluvium 

and colluvium that bury terrace gravel (fig. 5). Residual soils 

are also developed on the Tertiary marine sediments where the 

fluvial deposits are thin or eroded to thin lags. The residual 

soils may be welded with the overlying soil developed on the 

fluvial deposits.

The soils formed on the fluvial gravels demonstrate a 

clear progression of soil characteristics over time (fig. 6). 

Young soils are poorly consolidated and show little or no 

horizonation. Over time, clay-rich B horizons thicken and 

increase in secondary clay concentrations—from 0 to as much 

as 60 percent clay. As a result of increased clay concentration 

and secondary weathering, soil colors change from yellow brown (Munsell Color, 2009; 10YR), to reddish brown 

(7.5YR), to red (5YR) as deposits increase in age. The maximally developed B horizon also thickens from approximately 

60 to 120 cm (table 2, sheet 1). In soils of middle Pleistocene 

age, metasedimentary clasts are decomposed in the maximally 

developed B horizon. The oldest soil in the mapped area has a 

silica-cemented horizon or duripan (fig. 6).

In many actively incising river canyons, gravel-capped 

bedrock strath terraces are tens to hundreds of meters above the 

active channel and extend discontinuously for kilometers. Long 

recognized as recording the former river bed and floodplain 

elevation, strath terraces, when dated, provide a direct means 

of quantifying rates of vertical bedrock river incision and are 

thus vital to studies of geomorphology, active tectonics, and the 

effects of climate change (Molnar and others, 1994).

Strath terraces represent ancient and now-abandoned river 

floodplains that formed during periods of valley-bottom widening and planation (DeVecchio and others, 2012). Bull (1990) 

Figure 4. Photographs showing lag gravels and strath terraces. 

A, the active drainage is incising into the Tertiary marine 

sediments, leaving remnant lag gravel at the top of the eroded 

surface. B, strath terraces and associated soils preserve the same 

sequence that can be observed in Arroyo Seco today.

Lag gravel on strath terrace

Tertiary marine sediments

(Monterey Formation)

A

Lag gravel on strath terrace

Tertiary marine sediments

(Monterey Formation)

B
6 Quaternary Geologic Map of the North-Central Part of the Salinas River Valley and Arroyo Seco, California

suggested that lateral erosion and strath cutting are the dominant 

processes when streams are in dynamic equilibrium; that is, 

where river discharge and sediment load are in balance. Recent 

quantitative field studies and conceptual numerical models predict that strath terraces form in response to balanced or oscillating sediment supply, elevated sediment supply, and (or) periods 

of stable base level (Finnegan and Dietrich, 2011).

Explanations for why many incising rivers alternate 

between periods of dominantly lateral and dominantly vertical 

incision generally appeal either to temporal variation in baselevel stability or in the volume of coarse sediment supplied to 

rivers. For example, because gravel deposition in rivers armors 

the bed and suppresses vertical incision (Sklar and Dietrich, 

2004), some propose that lateral channel shifting and strath 

terrace formation is favored during climatic periods of higher 

sediment supply relative to river transport capacity, whereas 

vertical incision is favored when sediment supplies are low 

relative to transport capacity (Molnar and others, 1994). Alternatively, because base-level stability forces a river to remain 

at one elevation, others propose that either tectonic quiescence 

punctuated by periodic uplift or sea-level rise and fall superimposed on steady tectonic uplift (Pazzaglia and Gardner, 

1993) can generate flights of strath terraces.

Aggradation, during the onset of dry warm climatic cycles, 

results from increased sediment supply in response to transient 

vegetative conditions and consequent hillslope destabilization (Bull, 2007). Similar to aggradation events, strath terrace 

sediment ages may correlate to periods of dry, warm climate 

(DeVecchio and others, 2012). Increased sediment flux and 

decreased water discharge enhance lateral erosion rates and 

inhibit vertical incision. Subsequent incision, abandonment and 

strath terrace formation is inferred to occur during intervening wet climate intervals. In the western Transverse Ranges, 

where DeVecchio and others (2012) dated the strath terraces, 

they correlated strath terrace ages and aggradational events with 

environmental changes that are linked to global climate, indicating that climate rather than tectonics exhibits first-order control 

of depositional, denudational, and incisional processes.

Methods

The tools used to construct this geologic map were field 

work in combination with digital terrain analysis, stream gradient profiles, evaluation of published regional soil maps, and 

satellite imagery. Initial field work demonstrated that unique 

soils existed on both terrace and alluvial fan deposits, and that 

Arroyo Seco terrace surfaces could be correlated to alluvial 

fan deposits in the Salinas River valley.

Terrace-tread slopes were calculated from a 10-m digital 

elevation model using a geographic information system (GIS). 

Slopes less than 10 degrees were mapped and polygons were 

drawn around the individual surfaces. After measuring the 

elevation on each mapped terrace polygon, a longitudinal profile 

was constructed of all terrace surfaces along Arroyo Seco 

(fig. 7). Longitudinal profiles are measures of surface elevations along a stream gradient. Longitudinal profiles were also 

constructed on the other two major drainages in the map area—

Reliz Canyon and the Salinas River. Based on these profiles, terrace surfaces were correlated along Arroyo Seco. Twenty unique 

terraces were mapped from the point at which Arroyo Seco 

Figure 5. Buried terrace gravel exposed in road cut in Arroyo Seco. Buried terrace 

gravel was dated to determine the age of the buried deposit as well as the implied age of 

the alluvial fan.

Qfai2y

Qfai2o (?)

Fan alluvium

derived from

adjacent drainage

<65,000 years old

Terrace gravel

>65,000 years old
Methods 7

emerges from the narrow bedrock canyon to the intersection of 

Arroyo Seco with the Salinas River valley. Where Arroyo Seco 

enters the Salinas River valley, fluvial material is deposited and 

alluvial fans are formed. These fans were correlated to terrace 

surfaces based on elevation and soil development.

The soil map of Monterey County (Cook, 1978), in digital format [U.S. Department of Agriculture (USDA), 2009], 

was used to examine the soils developed on each of the unique 

terrace units. Soils were combined and simplified so that 

each map unit had a primary characteristic soil or set of soils 

(table 1, sheet 1). A preliminary geologic map that included 

both map units and soil types was used for field investigations 

to check units for consistency. Map-unit ages were initially 

assigned based on relative soil characteristics on Quaternary 

depositional units. Soils and bedrock units were observed in 

natural and existing manmade exposures. Field work focused 

on discrepancies between map units and soils—locations 

where soils were mapped on map units that seemed too old 

or too young based on the stratigraphic position of the unit. 

The final map product includes both the map unit and the soil 

developed on the unit (table 1, sheet 1). Note that soil units 

may be mapped on more than one map unit, and conversely, 

the same map unit may have more than one soil unit. Soil units 

are not mapped on oldest strath terraces (Qfo1–Qfo8) because 

original soil mappers did not recognize high terraces as being 

capped in alluvial gravel and only mapped residual soil.

Eight sediment samples were collected to determine 

age by infrared stimulated luminescence (IRSL) or optically 

stimulated luminescence (OSL) (S. Mahan, written commun., 

USGS, 2012) (table 1, sheet 1; table 3, sheet 2). All samples 

were collected at the base of the soil profile in the unweathered C horizon. Surface soils were sampled with the exception 

of AS10-9-6 that was sampled in a terrace gravel buried by an 

alluvial fan and exposed in a road cut in Arroyo Seco (fig. 5). 

See detailed geologic map for site locations and unit descriptions for discussion.

Holocene

middle Pleistocene

late Pleistocene

Grussified granite clast

early–middle Pleistocene

Duripan (Csim) = silica cemented soil horizon

Fluvent

Rincon

Placentia

Gloria

Chualar

late Pleistocene

Figure 6. Unique soils are developed on the terrace surfaces. After deposition and fluvial abandonment, soils develop redder and 

thicker B horizons. These B horizons become increasingly cemented over time. Once intact gravel weathers to grus, only scattered and 

larger clasts are visible on the surface of the deposit. These photographs are typical of common soils (names in yellow) developed on 

different-aged deposits (table 2, sheet 1).
8 Quaternary Geologic Map of the North-Central Part of the Salinas River Valley and Arroyo Seco, California

Figure 7. Stream gradient profile of terrace surfaces along Arroyo Seco. Different colored symbols represent different terrace surfaces.

Terraces are approximately parallel and range in elevation from 800 to 100 m above sea level.

0 5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000

Distance down Arroyo Seco drainage, in meters

Elevation, in meters

50

150

250

350

450

550

650

750

High small

terrace

remnants

Low broad

continuous

surfaces

Arroyo Seco stream gradient

EXPLANATION

Map units

Qfr1

Qfy1

Qfay

Qfai4

Qfai3

Qfai2y

Qfai2o

Qfai1

Qfo8

Qfo7

Qfo6

Qfo5

Qfo4

Qfo3

Qfo2

Qfo1
Acknowledgments 9

Discussion and Summary

Mapped strath terraces in the vicinity of Arroyo Seco 

record an erosion history from the early Pleistocene to latest Holocene. Alluvial fans adjacent to the range front and 

extending to the central Salinas River valley can be correlated 

to erosional surfaces in Arroyo Seco using soils and streamgradient profiles. Young terrace and floodplain deposits can 

be observed at and adjacent to Arroyo Seco and Salinas River. 

This suite of erosional and depositional units can be used to 

generalize the Quaternary history of Arroyo Seco.

Prior to the cessation of the deposition of the Paso Robles 

Formation (QTpr), an ancestral drainage was eroding the broad 

paleoerosional surface north of Arroyo Seco (fig. 1, sheet 1). 

The highest (oldest) deposits (Qfo1–Qfo5) record stream 

erosion and deposition prior to Arroyo Seco valley incision. 

Isolated now in the Sierra de Salinas, this paleoerosional surface 

forms a gently sloping, low-relief geomorphic surface northwest 

of Arroyo Seco. Like Arroyo Seco, this paleoerosional surface 

is constrained by basement rocks and lies on the more easily 

erodible Tertiary marine sediments. Bounded by a complex 

web of faults on all sides, these isolated erosional remnants 

are no longer linked to a large active drainage but record a 

paleodrainage now abandoned and superseded by the Arroyo 

Seco system. What remain as possible linked drainages are the 

greatly undersized Paloma and Piney Creeks (fig. 3). Individual 

erosional surfaces of the ancestral drainage (units Qfo1–Qfo5) 

tend to decrease in elevation in a northwestward direction. If 

higher surfaces truly are older than lower surfaces, the mapping 

suggests a general northwest trend in surface age, linking the 

paleoerosional surface to the headwaters of the Carmel Valley 

drainage (Tularcitos Creek). Today the Carmel Valley drainage 

is separated from the paleoerosional surface by a low divide. An 

alternative interpretation would link the paleoerosional surface 

to a drainage flowing southeast prior to the inception of Arroyo 

Seco, and surface age is convoluted by faulting that surrounds 

the surfaces, and (or) northwestward tilting. The fan shape of 

the paleoerosional surface, which extends to the northwest out 

of the map area, supports a southeast flow direction. The apex 

of the fan occurs approximately at the divide.

The timing of formation and isolation of strath deposits 

in Arroyo Seco, which postdate the formation of the 

paleoerosional surface, was likely controlled primarily by 

paleoclimatic events and secondarily by tectonics. Contributions to terrace formation that were driven by faulting would 

have to be attributed to the Rinconada fault because of the 

lack of evidence for Quaternary movement on the Reliz fault. 

Fault-related uplift may occur through the transfer of slip onto 

other faults in the Santa Lucia Range or by transpressive crustal 

shortening across the Santa Lucia Range (Dickinson, 2005). 

Net Neogene crustal shortening across the northern Santa Lucia 

Range of 10–12 percent (Dickinson, 2005) could result in 

2.2–2.7 km of bedrock uplift, corresponding to a 0.7–0.9 mm/yr 

tectonic component of the bedrock uplift (Ducea and others, 

2003). The absolute ages of the terraces in Arroyo Seco would 

be a valuable tool for establishing the magnitude of Quaternary 

uplift rates in the Santa Lucia Range. During this period of 

vertical movement, the level of the ocean has not remained the 

same. Changes in sea level are primarily produced by climate 

factors, but tectonics can also be a contributing factor. Taken 

together, these factors give rise to a complex pattern of continental erosion and deposition, which in this study are expressed 

as a sequence of strath terraces and associated alluvial fans in 

Arroyo Seco. DeVecchio and others (2012) have dated strath 

terraces at 110–100 ka, 50–35 ka, 26–20 ka and 15–4 ka in the 

southern California Transverse Ranges and correlated these depositional events to the onset of dry, warm climate cycles. The 

OSL and IRSL dates on seven individual terraces and associated 

alluvial fans in Arroyo Seco are approximately >120 ka, >65 ka, 

51–46 ka, 35 ka, 9 ka, and 2–1 ka (table 1, sheet 1). These 

dates generally fall within the range of ages reported from the 

Transverse Range, which has a similar climate to that of Arroyo 

Seco. DeVecchio and others (2012) support the interpretation 

that decreased vegetation and water during dry warm climate 

intervals causes increased sediment flux and increased lateral 

erosion. Incision and strath terrace formation is inferred to occur 

during wet climate cycles. In these similar tectonically active 

environments, climate rather than tectonics is the more influential factor in “deposition, denudation, and incision processes” 

(DeVecchio and others, 2012).

No terrace in Arroyo Seco is correlative with the older 

and higher paleoerosional surface. It can be assumed that the 

drainage, in its current location, did not exist when the paleoerosional surface was formed. Any alluvial fans in the Salinas 

River valley that may have been associated with the erosion 

of the paleosurface are long buried by younger fans. The 

oldest strath terrace remnants (Qfo6–Qfai2), confined to the 

Arroyo Seco drainage, are preserved as a wide strand across 

the now-confined valley. Based on the mapped distribution of 

the strath terraces, this older, much broader drainage was probably a braided stream, indicating a steep gradient with a high 

flow-velocity. Over time, the gradient decreased and the drainage became confined to a meandering stream (Qfai3–Qfr). 

Meandering streams flow in low gradient, low flow-velocity 

environments (Schumm and others, 2000).

Stacked and matching terraces across the Arroyo Seco 

drainage record an erosional history beginning in the early 

Pleistocene with the inception of the current drainage. Soils 

developed on these terraces can be used to constrain the timing 

of terrace formation and remnant fluvial deposition on eroded 

bedrock. Base-level change, driven by climate change, is the 

major contributor to terrace formation in Arroyo Seco.

Acknowledgments

This mapping was funded by the USGS National 

Cooperative Geologic Mapping Program. Constructive reviews 

of the draft by Lew Rosenberg and Marith Reheis greatly 

improved the quality of the map. Paco Van Sistine helped to 

add internal consistency to the attributes of the Geographic 

Information Systems data. Thank you to Tony Garcia 

(California State University, San Luis Obispo) for contribution 

of the panoramic photograph of the Arroyo Seco.
10 Quaternary Geologic Map of the North-Central Part of the Salinas River Valley and Arroyo Seco, California

DESCRIPTION OF MAP UNITS

Qcb Cut bank (latest Holocene)—Steep exposed bluffs on active drainages

Qfr2 Fluvial and minor eolian deposits (latest Holocene)—Active channel, floodplain including gravel and sand bars, 

and modern dunes at and adjacent to the distal floodplain of Arroyo Seco and adjacent to the Salinas River 

with little or no vegetation. Soils are gravelly to sandy Entisols with no horizon development

Qfr1 Fluvial deposits (latest Holocene)—Terrace and floodplain deposits, <1 m above the active channel of Arroyo 

Seco and Salinas River with sparse vegetation. Soils are sandy Entisols with no horizon development (see 

Fluvent photo, fig. 6)

Qfy2 Fluvial deposits (late Holocene)—Terrace and floodplain deposits <2 m above the active channel of the floodplain 

of Arroyo Seco and Salinas River. Soils are fine-loamy Mollisols with an A/C profile

Qfy1 Fluvial deposits (late Holocene)—Terraces adjacent to Arroyo Seco, <3 m above the active channel, prograding 

to minor terrace remnants on the floodplain where unit may interfinger with Qfr1. Within Arroyo Seco, 

deposit often hosts large, established sycamore trees and unit Qfy1 has been dated at about 1,000 years old 

(table 3, sheet 2; see Fluvent photo, fig. 6). Terraces also occur as remnants <7 m above the active channel of 

the Salinas River. Soils are sandy Entisols with an A/C profile

Qfay Fluvial and alluvial deposits (middle to early Holocene)—Strath terraces on Arroyo Seco that prograde to 

alluvial fans. Alluvial deposits enter the Salinas River valley and bury older fan deposits and are dated at 

9,570 ± 650 years old (table 3, sheet 2). Alluvial fans have been eroded by younger Qfy1 and Qfy2 deposits. 

Inset alluvial terraces of Qfay age also occur along Chalone Creek on the east side of the Salinas River valley. 

No Qfay is mapped south of Arroyo Seco. Soils are coarse-loamy Mollisols with an A/C profile

Qfai4 Fluvial and alluvial deposits (latest late Pleistocene)—Alluvial fans and terraces, adjacent to both Arroyo Seco 

and Salinas River. Deposit is often a thin mantle <2 m thick over Qfai3 at and adjacent to the range front. 

Qfai4 was dated but the age, 8,770 ± 350 years old (table 3, sheet 2), is not consistent with the soil formed on 

the deposit and the deposits close proximity in depositional timing to Qfai3. The deposit is probably between 

20,000 and 30,000 years old. Qfai4 occurs as a strath terrace on the east side of the Salinas River, and in 

Arroyo Seco about 30 m above the active floodplain. Exposures of Qfai4 extend only about 7 km west of the 

intersection of Arroyo Seco with the Salinas River valley. Soils are clayey Alfisols with an A/Bt/C profile. 

Bt horizon has a brownish hue (10YR) with about 45 percent clay. High clay content in maximally developed 

B horizon (table 2, sheet 1; Rincon soil series) may result from contributions of fine-grained overbank flood 

deposits. Secondary carbonate is infrequently observed in the C horizon (see Rincon photo, fig. 6)

Qfai3 Fluvial and alluvial deposits (latest late Pleistocene)—Alluvial fans and strath terraces associated with Arroyo 

Seco and Reliz Canyon. Qfai4 and Qfai3 are the dominant inset terraces along the meandering Arroyo Seco 

and very little time separates the deposition of these two units. Where Qfai3 forms strath terraces in Arroyo 

Seco it is about 40 m above the active drainage and where it forms alluvial fans it is about 10 m above Qfai4. 

Qfai3 alluvial fans are buried by unit Qfay near the confluence of Arroyo Seco and Reliz Canyon. Remnants 

of the Qfai3 strath terraces extend the entire length of Arroyo Seco. Qfai3-age deposits surround the abandoned oxbow lake near the canyon mouth of Arroyo Seco, near the western map edge, suggesting that the 

meander sweep was abandoned after the erosion of Qfai3. Qfai3 is dated at 35,350 ± 3,680 years old (table 3, 

sheet 2). Soils are predominantly loamy alluvial Mollisols with an A/Bt/C profile. Bt horizon has a brown 

(10YR) to reddish hue (7.5YR) with 25–35 percent clay (See Chualar photo, fig. 6)

Qei Eolian deposits (latest late Pleistocene)—Eolian sand dunes modified to terraces, about 30–40 m above the active 

floodplain of the Salinas River. Deposit is east of the Salinas River, and has been eroded by Qfai4-age deposits. This deposit is also southeast of the town of King City, just east of the mapped area. Soils are loamy sandy 

Alfisols with an A/Bt/C profile. Bt horizon has a brownish (10YR) to reddish hue (7.5YR) with 15 percent clay

Qfai2y Fluvial and alluvial deposits (latest late Pleistocene)—Alluvial fans and strath terraces. A Qfai2y terrace in 

Arroyo Seco is dated at 50,640 ± 2,980 and 45,960 ± 1,840 years old (table 3, sheet 2). The erosional event 

associated with the formation of Qfai2y was indirectly dated at <65,000 years old because a Qfai2y alluvial 
DESCRIPTION OF MAP UNITS 11

fan buried a terrace gravel that was dated at >65,000 years old (fig. 5). Qfai2y is about 80 m above the 

modern drainage. Qfai2y forms a predominant range-front alluvial fan south of Arroyo Seco. Soils are loamy 

alluvial Mollisols with an A/Bt/C profile. Bt horizon has a brownish hue (10YR) with about 35 percent clay

Qfai2o Fluvial and alluvial deposits (latest late Pleistocene)—Strath terraces in Arroyo Seco. Terraces are typically 

remnants located above the modern drainage and large strath terraces within the meandering drainage. Buried 

Qfai2o terrace deposits were dated at >65,000 years old. Soils are the same as those on Qfai2y indicating 

very little time between the abandonment of Qfai2o and Qfai2y

Qfai1 Fluvial and alluvial deposits (early late Pleistocene)—Strath terraces in Arroyo Seco formed on the Tertiary marine 

sediments and Paso Robles Formation. Terraces form dramatic erosional features on both the east and west sides 

of the Salinas River valley, including the range front where Arroyo Seco intersects the broad valley. These terraces, 

and all older terraces, lack the general meandering nature of the younger terraces and were created in a much 

broader alluvial floodplain than today. This broader floodplain was probably a braided drainage system suggesting a higher velocity drainage than the current meandering system. Soils are typically coarse-loamy Mollisols and 

Alfisols developed on granitic alluvium with an A/Bt/C profile (see Placentia photo, fig. 6). The Bt horizon has a 

brown (10YR) to darker reddish-brown hue (5YR). Qfai1 also includes silty and clayey residual soils, developed 

on both Tertiary marine sediments and the Paso Robles Formation. These soils have an A/C profile

Qao Alluvial deposits (late middle Pleistocene)—Alluvial fans flanking range front north of Arroyo Seco, extending 

north of the mapped area on the east and west sides of the Salinas River valley. Soils are coarse-loamy Alfisols 

developed on granitic alluvium with an A/Bt/Csim profile. Silica cementation in the C horizon, called a duripan (Csim), is extremely hard when dry and does not slake during prolonged soaking in water. The Bt horizon 

has a reddish-brown (7.5YR) to darker reddish-brown hue (5YR) (see Gloria photo, fig. 6)

Qfo8–1 Fluvial deposits (middle to early Pleistocene)—Strath terraces Qfo8–Qfo1 on Tertiary marine sediments. Stranded or isolated Arroyo Seco terrace treads capped by rounded granitic cobbles to boulders. Terraces are on both 

sides of the drainage. All surfaces have a less than 10 degree slope. Individual remnants of units Qfo8–Qfo1

are correlated by stream gradient profile interpretations (fig. 7). Older surfaces (Qfai1 and older) record a 

broader, high flow-velocity, steep gradient, braided stream in Arroyo Seco that has subsequently evolved into 

the current meandering drainage

QTpr Paso Robles Formation (middle Pleistocene to Pliocene)—Unconsolidated to poorly consolidated heterogeneous 

coarse sand and gravel, as well as interbedded finer sand, silt, and clay. Coarse fragments are primarily siliceous shale of the Monterey Formation. Environment of deposition is fluvial and includes some debris flows 

and lacustrine deposits. Datable tephra layers are interbedded with clastic strata. Tephra samples from the 

Paso Robles Formation in the Gabilan Range include the Nomlaki Tuff (3.4 ± 0.4 Ma) and the Lawlor Tuff 

(4.1 ± 0.2 Ma) (Sarna-Wojcicki and others, 1991). The Paso Robles Formation is exposed in the hills on the 

east and west sides of the Salinas River valley. On the east, it overlies unit }|ub (undifferentiated basement) 

and on the west Tertiary marine sediments (Tms). Unit is commonly capped by erosional strath terraces. 

Ranges in thickness from 60 to 600 m

Tsa Shallow nearshore marine regressive sediments (Pliocene)—Informal sandstone of San Ardo. Sandstone, light 

gray, friable, medium grained, vaguely bedded, contains shell fragments. Unit occurs along the range front 

where Arroyo Seco intersects the Salinas River valley and in Reliz Canyon

Tpr Pancho Rico Formation (Pliocene to Miocene)—Siltstone, light gray diatomaceous mudstone to fine-grained 

silty sandstone, vaguely bedded. Shallow-marine, estuarine, and lagoonal sediments. In the map area, 

Tms occurs on the west side of the Salinas River valley, however, south of the map area, the Pancho Rico 

Formation flanks the east side of the Salinas River valley

Tms Tertiary marine sediments (Miocene to Paleocene)—Primarily Monterey Formation. Siliceous shale and sandstone. Shale is white to gray, thinly bedded, soft to hard, platy, contains thin, hard layers of limestone. Abundant fish fossils. Sandstone is light gray to tan, massive to thick-bedded, medium-grained, arkosic. Includes a 

small outcrop of Paleocene to Eocene Reliz Canyon Formation south of Arroyo Seco

}|ub Undifferentiated basement rocks (Mesozoic to Paleozoic)—Plutonic basement, primarily granitic rocks, and 

metasedimentary rocks, mostly schist and gneiss. Metasedimentary rocks are intruded by granitic rocks. 

Rounded granitic rocks derived from unit occur as field stones capping strath terraces (fig. 7, sheet 1)
12 Quaternary Geologic Map of the North-Central Part of the Salinas River Valley and Arroyo Seco, California

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Taylor and Sweetkind—Quaternary Geologic Map of the North-Central Part of the Salinas River Valley and Arroyo Seco, California—Scientific Investigations Map 3260