March 2009 LIP of the Month

The Lovejoy Basalt: A (fairly-) Large Igneous Province in Northern California

Rachel Teasdale, California State University Chico, Department of Geological & Environmental Sciences,

Figure 1: Location map of Lovejoy Basalt, east of the Cascade arc and southwest of the Snake River Plain basaltic centers, and approximately 500 km south-southwest of Columbia River Basalt vents. Modified from Garrison, et al., 2008.


The Miocene Lovejoy Basalt flows are a series of lavas exposed in northern California in large outcrops that form landmark structures as well as discontinuous layers buried in the subsurface across the Sacramento Valley. Lavas are thought to have erupted from vents located at Thompson Peak, near Susanville CA, east of the Lassen Volcanic Center and southeast of Mount Shasta (Wagner et al., 2000; see figures 1 and 2). Similarities in age and composition with Columbia River Basalts (CRB) flows have led to the suggestion that Lovejoy Basalts have similar origins as the CRBs (Garrison et al., 2008).

The Lovejoy Basalt fits the Large Igneous Province criteria established by the IAVCEI LIP Commission (, accessed 2009) in that they cover large areas and are not considered to be related to "normal" sea floor spreading or subduction related events.

Age of Lovejoy Basalt

Radiometric dating of the Lovejoy Basalt has been challenging because lavas are fine grained and have undergone Ar loss associated with glass hydration (Garrison et al., 2008). Fine grained groundmass of olivine and plagioclase crystals, the latter of which have high Ca (up to 16% CaO, An82) and low K (average 0.52 % K2O), which contributes to difficulty in obtaining Ar dates (Garrison, et al., 2008). Recently published 40Ar/39Ar ages cluster at approximately 15.4 Ma (Garrison et al., 2008), which correlates well with previous ages that range 15.5-16.4 Ma (Page et al., 1995 and Garrison, 2004) and previous K-Ar ages of 14-17 Ma (Wagner et al., 2000). Paleomagnetic signatures indicate that the flows were erupted over a relatively short timeframe, likely within a few hundred to a few thousand years (Coe, 2005). Given a range of 500 to 5000 years of eruptions, the estimated magma supply rates for all Lovejoy Basalts (0.3.03 km3/yr) are on par with rates of CRB lavas (0.17 km3/yr, Camp et al., 2003), also similar to Kilauea magma supply rates (0.18 km3/yr, from Pyle, 2000). These volumes and magma supply rates suggest the emplacement of the Lovejoy Basalts was an important geologic event in Northern California during the Miocene.


Lovejoy Basalt flows traveled nearly 250 km southwest across the Sacramento Valley to Putnam Peak, near Lake Berryessa and Vacaville, California (figure 2). The total volume of the Lovejoy Basalt is approximately 150 km3 over an area of approximately 128,000 km2, as reported by Durrell (1987). Outcrops of Lovejoy Basalts are exposed in the Sierran Foothills at Red Clover Creek, Walker Plains and Little Grass Valley Reservoir, as well as at Table Mountain (Coon Hollow site) near Oroville, several sites near Chico (Isom Quarry, Bear Hole sites) further west, across the Sacramento Valley at Black Butte Reservoir near Orland, and near Vacaville at Putnam Peak. Lovejoy Basalt flows of Table Mountain near Oroville are preserved as an approximately 30 km2, 200' high plateau that is home to locally famous wildflowers and vernal pools.

Figure 2: Location map of Lovejoy Basalt outcrops of northern California discussed in text. Black line shows locathin of the line of cross section shown in figure 3. Modified from Reioux and Teasdale, 2008.

Well logs intersect subsurface Lovejoy Basalt flows across the Sacramento Valley where lavas unconformably overlie the Eocene Ione Formation and the Great Valley Sequence. Subsurface localities of Lovejoy Basalts are detected in drill core with thicknesses up to 75 m, similar to surface exposure thicknesses on the west side of the Sacramento Valley (at Orland Buttes and Putnam Peak shown in figure 2) and are shown in figure 3 (compiled by D. Spangler and others at the California Department of Water Resources, Northern District, Groundwater Section).

Figure 3: Cross section showing Lovejoy Basalt locations (pink) in the subsurface in northern California. Constructed from DWR data using multiple well logs across the nothern Sacramento Valley. Line of cross section is shown by black line in figure 2. From Debbie Spangler, courtesy DWR Northern District Groundwater Section and modified from Teasdale and Spangler, 2008.

Field Exposures

Stratigraphic sequences of Lovejoy Basalts include up to 9 individual flow units identified at Red Clover Creek (Wagner 2000), four of which are shown in figure 4. The total thickness at Red Clover Creek is approximately 250 m (Garrison et al., 2008), which represents the maximum known preserved thickness of a sequence of flows.

The full thicknesses of individual flow units are commonly obscured but are approximately 5-10 m thick at Red Clover Creek and are reported as up to 20 m thick at Black Butte (Siegel 1988). One flow at Table Mountain located at Coon Hollow is approximately 60 m thick, but is unusually thick and appears to be topographically controlled, causing lava to pond, forming an unusually thick section. Individual flows of Lovejoy Basalt at the farthest known extent, Putnam Peak, are approximately 10 m thick, and also include pervasive columnar jointing, approximately 10 cm across.

Figure 5: Typical "rubbly" appearance of jointed Lovejoy Basalt outcrop along Big Chico Creek in Chico, California.

Most outcrops are entirely or partially constructed of basaltic columns (or rubbly remains of columnar material). Typical columns are approximately 10-20 cm across, but Little Grass Valley Reservoir outcrops include much larger columns, up to nearly 1 m diameter (see figures 5 and 6a, b).

Figure 6a: Columns at Little Grass Valley Reservoir, R Teasdale

Figure 6b: Little Grass Valley Reservoir: wide and narrow columns in two flow units

Basalt Characteristics

Fresh samples are typically dark black, nearly aphanitic basalt that weathers to dark to medium brown. Phenocrysts of subhedral plagioclase occur in some flows and garnet xenocrysts have been reported for flows at Red Clover Creek (Garrison et al., 2008). Groundmass mineralogies are dominated by plagioclase (< 1mm), up to 50% of total crystallinities. Olivine and pyroxene crystals (<.5 mm) each make up less than 10% of the total groundmass. Most samples lack oxide phases.

Figure 7: Major element compositions of Lovejoy Basalts (from Garrison 2004 and Teasdale & Spangler 2008) along with regional basalts for comparison. CRB-Reidel, 1983 and Wolff et al., 2008; SRP-Sims, 2000; N Nevada Rift-Fitton, pres comm.; Steens-Camp et al., 2003; Lovejoy-Garrison, 2004 and Wilson et al., 2005.

The Lovejoy Basalt flows have nearly homogeneous major element compositions (figure 7) that cluster along the boundary between alkaline and sub-alkaline basalt, basaltic andesite, trachybasalt, and basaltic trachyandesite, but vary by less than 2.5 % SiO2 and less than 1% MgO.

Major element variations indicate Lovejoy samples are trachy-basalts and basaltic trachyandesites. Garrison and others (2008) suggest compositional similarities of Lovejoy Basalts with Columbia River basalts so several western US basalts are plotted for comparison in figure 7 (from Teasdale and Spangler, 2008). More detailed discussion of the petrologic characteristics of the Lovejoy Basalts and possible origins is given below.

Figure 8: Anorthite compositional range for Lovejoy Basalt sites in order of distance from source (Thompson Peak, near Susanville, California. RCC Red Clover Creek, LGV-Little Reservoir. All locations shown in figure 2.

Mineral Compositions

Groundmass plagioclase compositions of near-source samples of the Lovejoy Basalt range from ~ 37-60% An. Putnam Peak samples, located nearly 250 km away, range from 39-54% An (figure 8 from Reioux and Teasdale 2008). This minimal variation suggests similar crystallization histories (rates and timescales) for the two localities, which are also the two furthest separated sites. In contrast, Table Mountain and samples collected from drill core (Pentz locality at the NE end of figure 3) vary more widely (11-57% and 28-81% An, respectively) which may be related to lava ponding at each of those sites, which would have kept flows insulated and increased the duration of cooling and crystallization, increasing the extent of groundmass crystal differentiation during emplacement.

Figure 9a: Proportions of crystals (by area) generally increase with distance from source. Figure 9b: Sizes of groundmass crystals tend to increase with distance from source. Symbols: RCC - Red Clover Creek, LGV - Little Grass Reservoir, WP - Walker Plain, TMtn - Table Mountain, PDC - Pentz Drill Core, BB - Black Butte Reservoir, PP - Putnam Peak.

Groundmass Crystallinities

Groundmass crystals are dominated by plagioclase crystals approximately 100-200 μm long. Proportions of groundmass plagioclase are estimated for several sites across northern California and measured using digital image analysis (Reioux and Teasdale 2008). In general, the number of plagioclase crystals increases distance from source (figure 9a), which reflects that lavas that traveled farther from the source, nucleated more crystals prior to emplacement. Similarly, crystal size tends to increase with distance from source. This trend is consistent with more time for crystal growth as lavas flow greater distances (figure 9b).

The Table Mountain, Coon Hollow section samples have groundmass crystal sizes larger than predicted by a linear trend of crystal size vs. distance from source (figure 9b). The section of flows at Table Mountain is very thick (approximately 61 m), which is likely due to flows pooling there, during flow emplacement. That overly-thick section of lava would have provided greater insulation for crystals to grow than at other areas sampled and measured, explaining the "oversized" groundmass crystals at Table Mountain.

Variations of the groundmass crystallinities for three stratigraphic sections of multiple flow units at Oroville, Table Mountain, Little Grass Valley Reservoir and Red Clover Creek have been measured (Reioux & Teasdale, 2008 and Teasdale 2007 respectively). In each of the three cases, crystal sizes generally increase up-section and become less complex. Crystal sizes and shapes may be useful in characterizing the relative time intervals between flow unit emplacement. Smaller, more complex crystals reflect faster cooling and larger, blockier crystals represent slower cooling. Flows would cool more slowly if insulated by subsequent flow units, making flow units with larger, blockier crystals good representatives of relatively increased time between emplacement of flow units.

Most flow units within sections of multiple flow units suggest they were well insulated by subsequent flow units. This is consistent with continued emplacement of additional flows, which would be required for rapid emplacement of the entire Lovejoy Basalt volume. The estimated total volume of Lovejoy Basalts is 150 km3, approximately an order of magnitude smaller than individual major flows of the Grande Ronde (CRB). Paleomagnetic signatures indicate that the flows were erupted over a relatively short timeframe, likely within a few hundred to a few thousand years (Coe, 2005). Given a range of 500 to 5000 years of eruptions, the estimated magma supply rates for all Lovejoy Basalts (0.3-.03 km3/yr) are on par with rates of CRB lavas (0.17 km3/yr, Camp et al., 2003), also similar to Kilauea magma supply rates (0.18 km3/yr, from Pyle, 2000). These volumes and magma supply rates suggest the emplacement of the Lovejoy Basalts was an important geologic event in Northern California during the Miocene.

Figure 10: Extent of Yellowstone plume head underlying lithosphere in western North America. Location of Lovejoy Basalt source area (Thompson Peak) shown with star. Modified from Camp and Ross 2004.


Lovejoy Basalt lavas are tholeiitic with tightly constrained major element compositions (e.g. 3.8-4.7% MgO and see figure 7). Previous workers have connected the timing, spatial distribution, and compositions of Lovejoy Basalt lavas with Columbia River Basalts (e.g. Wagner et al., 2000 and Garrison et al., 2008). Below is a brief summary of characteristics considered: Timing: Eruptions of the (~15.4 Ma) Lovejoy Basalt (Garrison et al., 2008) are approximately contemporaneous with those of the Grande Ronde basalts (16.5-15.6 Ma, from Reidel et al., 2003). Spatial Distribution: Spatially, connecting the Lovejoy Basalt with the track of the Yellowstone plume is also plausible. The extent of the Yellowstone Plume is shown in figure 10 as having arrived in SE Oregon at ~ 16.6 Ma (Camp & Ross, 2004). The convecting plume head then spread along the base of the lithosphere, resulting in decompression-induced melting that produced Steens Basalts and even more voluminous magmas of Grande Ronde CRBs (Jordan et al., 2004). Lateral spreading of magma (of CRBs) occurred along preferential pathways, predominantly towards the north, but also east and southeast (Camp & Ross, 2004), which may permit inclusion of the Lovejoy Basalts within the reach of plume material.

Fissures of the Lovejoy Basalts at Thompson Peak are aligned northwest, parallel with those of the CRBs, suggesting similar stress regimes during eruption of both magmas (Wagner et al. 2000). Later work calls on a "...rapid migration of material from the Yellowstone mantle plume head..." to explain the Lovejoy Basalt as a "...southern extension of mid-Miocene flood basalt volcanism..." (Garrison et al., 2008).

Compositions: Lovejoy Basalts major element compositions and isotope ratios overlap with CRB lavas, particularly with Grande Ronde and Imnaha lavas (e.g. Wolff et al., 2008), consistent with similar source magmas (Teasdale 2007, Garrison et al., 2008). More recent trace element ratio data (Teasdale and Spangler, 2008) indicates distinctions between the Lovejoy Basalt and CRB lavas (e.g. Y/Nb, and Ba/La) that show that the Yellowstone plume model does not adequately explain the magmatic origins of the Lovejoy Basalts. Detailed analysis of trace element ratios provides good evidence for a subduction-related fluid component, which distinguishes the petrogenesis of Lovejoy Basalt magma from the Yellowstone plume source of the CRBs (Teasdale and Spangler, 2008 and Teasdale, in preparation).


The Lovejoy Basalt flows of northern California were an important regional event that produced extensive lava flow sequences across northern California. Flow emplacement appears to have occurred over a short time period around 15 Ma (Coe et al., 2005), which was strongly influenced by paleotopography (Wagner et al., 2000). Affiliation of Lovejoy Basalts and CRB magmas is consistent with the timing, and is plausible in terms of spatial distributions of each, but compositions require refinement of interpretations of source and petrogenesis of the Lovejoy Basalt magmas. These and related topics of flow emplacement and compositional variability are subjects of ongoing investigations.


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