Compositional and diagenetic control of bed- to formational-scale deformation in siliceous sedimentary rocks, Santa Maria Basin, California
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Published:September 26, 2022
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Yannick Wirtz*, Richard J. Behl, 2022. "Compositional and diagenetic control of bed- to formational-scale deformation in siliceous sedimentary rocks, Santa Maria Basin, California", Understanding the Monterey Formation and Similar Biosiliceous Units across Space and Time, Ivano W. Aiello, John A. Barron, A. Christina Ravelo
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Rock properties play a critical role in dictating styles of deformation at all spatial scales, yet the effect of changes across and within diagenetic transition zones has been little studied, despite profound impact on resulting mechanical stratigraphy. Our analysis of the variation of fold strain at map scale and outcrop scale of the highly siliceous Monterey and Sisquoc Formations in the southern Santa Maria Basin, California, provides insight into the interplay among deformation, diagenesis, and rock composition. Diagenetic modification of these rocks has created intervals with high interstratal and interformational contrasts in competence. Map-scale analysis showed large variation in fold strain within the same area, with shortening values ranging from 5.5% to 21.1% between siliceous formations of different diagenetic grade and competence. Apparent shortening in the competent, diagenetically altered, thinly bedded Monterey Formation is twice as high as that in the overlying highly porous, diatomaceous, more massive Sisquoc Formation. The large difference in measurable apparent shortening suggests that the same amount of actual strain was chiefly accommodated by folding in the Monterey Formation versus horizontal compaction in the Sisquoc Formation, since there is no evidence of a detachment between the units. Strain analysis at outcrop scale provided insight into the ways in which both units express such different shortening ratios without having an unconformity or detachment fault between them.
Reconstruction of the structural evolution of fold-and-thrust belts requires identification of deformation mechanisms and quantification of shortening. Balanced cross sections account for translational and rotational strain components by integrating surface and subsurface data across fold-and-thrust belts generally at regional scales. The construction of these sections produces geometric and kinematic models, illustrates the deformational history, and predicts structural traps for petroleum prospects in basin exploration (Suppe, 1980; Namson and Davis, 1988a, 1988b, 1990; Mitra and Namson, 1989). Yet, balanced cross sections have limitations because they do not account for the different mechanical behaviors of rocks and their deformation mechanisms at finer scales that may affect structural development in local regions within fold-and-thrust belts.
Studies focusing on different scales, lithologies, and environmental conditions (e.g., pressure, temperature) have demonstrated significant differences in deformational pattern (Snyder, 1987; Behl, 1992), amount of shortening (Koyi et al., 2003), variation in strain and structural style along strike (Carbonell et al., 2013), and interpretation of the kinematics and mechanics of emplacement of thrusts (Mitra, 1984; McQuarrie and Davis, 2002). Therefore, strain information within a single tectonic setting that is derived from varied scales and lithologies with contrasting physical properties may significantly improve understanding of the different local structural styles.
Due to their unique stepwise diagenesis, siliceous sedimentary rocks can exhibit extremely heterogeneous and contrasting physical properties (Snyder et al., 1983; Weller, 2018). Siliceous sediments are initially deposited as highly porous oozes of diatoms or radiolarians before undergoing significant and complex mechanical modification during burial diagenesis, resulting in variations in physical rock properties (Isaacs, 1981; Chaika and Dvorkin, 2000). This evolution provides the opportunity to study the magnitude of strain variation across mechanically contrasting lithologies in fold-and-thrust belts at map scale and outcrop scale.
In this study, we quantified and analyzed variations in strain in the petroliferous Monterey and Sisquoc Formations in the southern Santa Maria Basin, California, because they consist of siliceous strata at distinct diagenetic stages. The results presented herein provide an explanation for the ways in which distinct deformational styles between contrasting-competence siliceous diagenetic rocks at formational and outcrop scale can exist without a detachment between them, and they provide potential for refined prediction of geologic structures in the subsurface.
The part of the southern Santa Maria Basin examined in this study exposes siliceous strata of the Monterey Formation and overlying Sisquoc Formation over 340 km2. Two areas of positive relief were chosen to investigate strain in the southern Santa Maria Basin. The two study areas are termed herein as the Lompoc–Santa Rosa fold belt to the south and the Lompoc-Purisima anticline fold belt to the north (Figs. 1A and 1B).
Their location is within a structural province of regional contraction that formed four anticlinal trends, 15–80 km in axial length and 2–3 km in structural relief. Regional-scale balanced cross sections interpret the anticlinal trends to be fault-bend and fault-propagation folds resulting from thrust ramps off thrust flats and a regional detachment at 11–14 km depth (Fig. 1B; Namson and Davis, 1990).
In the study areas, the fold style diverges significantly from the regional fold belt of the Santa Maria Basin. The regional folds in the Santa Maria Basin have wavelengths of 5–10 km and axial lengths of up to 40 km (Figs. 1A and 1B). In contrast, the study areas are composed of localized east-west–oriented folds with wavelengths ranging from 0.1 km to 3 km and axial lengths ranging from 0.5 km to 10 km. These folds are almost entirely confined to the upper Monterey and Sisquoc Formations, and just a few structures with regional axial lengths of over 10 km extend into adjacent units of younger or older age outside of the study area (Fig. 1B). Only a few regional folds underlie the highly folded Monterey Formation. First-order folds are >10 km in axial length and part of major structures that involve older and younger rocks. Second-order folds are 0.5–8 km in axial length and represent the majority of map-scale folds (Figs. 1A and 1B). Third-order folds are 0.1–0.5 km in axial length and occur in smaller sets. Second- and third-order folds represent the majority of folds and are confined to the upper Monterey and Sisquoc Formations (Figs. 1A and 1B).
Rocks of the upper, thinly bedded siliceous member of the Monterey Formation in the Lompoc–Santa Rosa fold belt display intraformational deformation at outcrop scale (Snyder, 1987; Gutiérrez-Alonso and Gross, 1997). This deformation includes a variety of different types of detachment folds and fault-propagation folds (Snyder, 1987) that are interpreted to be the result of blind thrusts splaying off detachment horizons at depth and bedding-plane detachments themselves folded progressively during deformation (Gutiérrez-Alonso and Gross, 1997). Rocks in the Lompoc-Purisima anticline fold belt have not been studied before, and map-scale folding between the Monterey and Sisquoc Formations was examined in this study.
Monterey and Sisquoc Formations
The biosiliceous Monterey and Sisquoc Formations were deposited during the middle Miocene to early Pliocene (Barron, 1986), prior to widespread regional contraction, and have accommodated tectonic shortening with contrasting deformational styles in different locations and stratigraphic intervals (Snyder et al., 1983; Snyder, 1987; Gutiérrez-Alonso and Gross, 1997). Even within a single outcrop, and therefore by definition in areas with identical stress histories, beds can record widely different structures and individual styles of tectonic deformation. This is due to a complicated interplay between the deformational and diagenetic histories of the bedded siliceous sedimentary rocks that affected their deformational style (Snyder et al., 1983; Snyder, 1987; Behl, 1992, 1999). The Monterey Formation and the overlying Sisquoc Formation in the study area consist of siliceous sediments in a complete spectrum of diagenetic stages. The degree of the diagenesis was controlled by the initial sediment composition and burial history, principally temperature increase with depth (Fig. 2). The thermal gradient in this part of the Coast Ranges is high, ranging from 40 °C/km in shallow sediment to 60–82 °C/km in the diatomaceous Sisquoc Formation, as measured in wells located within uplifted parts of the Lompoc-Purisima anticline fold belt study area (Williams et al., 1994). In continuously buried locations of the northern Santa Maria Basin, the transition from opal-A to opal-CT is heterogeneous on a bed-to-bed basis due to clay and carbonate content, but it generally occurs between 38 °C and 54 °C and less than 1 km of burial depth (Pisciotto, 1981).
This mineralogic change enables dramatic variation in rock properties, even within a single outcrop, as the silica phase progresses from opal-A (diatomaceous sediments composed of X-ray amorphous silica) to opal-CT (metastable silica composed of poorly ordered, hydrous cristobalite and tridymite) to quartz (Isaacs, 1981). In this area, the stratigraphically higher Sisquoc Formation is composed chiefly of opal-A phase diatomaceous sediments with gradually alternating detrital content (Fig. 3). In contrast, the lithologies in the underlying upper Monterey Formation are highly heterogeneous and progressively composed of more opal-CT phase rocks of higher competence down section (Fig. 3). For simplicity in this structural study, all porous opal-A diatomaceous rocks will be termed as diatomite, and all hard to brittle opal-CT diagenetic rocks will be termed as chert/porcelanite, regardless of variations in their detritus content. Additionally, although the Sisquoc Formation does consist of diagenetically altered, opal-CT phase siliceous rocks elsewhere or when deeply buried in the subsurface, it consists predominant of opal-A diatomaceous rocks in this study area and is considered as the diatomite unit for this comparative analysis.
Layer-to-layer variation in structural behavior based on the distinct physical properties of the rocks has been described as mechanical stratigraphy (Gross et al., 1997), which, in the case of siliceous sediments, results from the combination of initial sediment composition and diagenetic modification. In addition to the bulk difference in diagenetic phase, the two formations differ in mechanical stratigraphy: The upper Monterey Formation is largely composed of thinly interbedded intervals of incompetent, porous diatomite and competent, hard, and brittle chert/porcelanite, whereas the overlying Sisquoc Formation is composed of thickly bedded, highly porous diatomite (Fig. 3). The great mechanical anisotropy between the Sisquoc and the upper Monterey Formations provided the basis for development of different deformational mechanisms in close proximity, including both consumption of strain via volume reduction in diatomites (pure shear) and complex interplay among flexural slip, folding, and faulting within interbedded diatomite and chert/porcelanite (simple shear).
Map-Scale Strain Analysis
Tectonic shortening was quantified in the upper Monterey and Sisquoc Formations in order to address the impact of lithology and diagenetic state on deformational behavior and better understand variation in strain intensity across the Lompoc–Santa Rosa fold belt and Lompoc-Purisima anticline fold belt. Nineteen cross sections across the upper Monterey and Sisquoc Formations in the Lompoc–Santa Rosa fold belt, and nine cross sections across the Lompoc-Purisima anticline fold belt were constructed to generate the key data set for shortening estimates and fold geometry description (Fig. 1). Three representative cross sections illustrate the line-length balancing and creation of dip domains with kink geometries to estimate shortening rates (Fig. 4), similar to the cross-section construction by Namson and Davis (1990) (for methodologies for shortening estimates, see results table and all cross sections in Wirtz  and Supplemental Material1).
The data show large variations in strain along the strike of the fold axes over short distances (Fig. 5). In the Lompoc–Santa Rosa fold belt, shortening values range from 5.5% (profile 6) to 21.1% (profile 10), with three areas of high strain separated by areas of low strain, where shortening significantly decreases over distances of just a few kilometers. In the Lompoc-Purisima anticline fold belt, shortening values range from 4.7% (profile 21) to 16.8% (profile 28), increasing in magnitude from the western part to the eastern part. As in the Lompoc–Santa Rosa fold belt, significant increase in strain occurs over just a few kilometers along strike (Fig. 5). The Sisquoc Formation displays only half as much of the measurable fold-related shortening as the Monterey Formation, with relative contributions to the total shortening of 0.52:1 in the Lompoc–Santa Rosa fold belt and 0.48:1 in the Lompoc-Purisima anticline fold belt (see Supplemental Material; Wirtz, 2017).
In both study areas, strain calculations showed a positive relationship between the amount of total shortening and the relative amount of Monterey Formation exposure compared to the Sisquoc Formation. Transects with mostly Monterey Formation exposures showed the highest shortening rates, and transects with mostly Sisquoc Formation exposures showed the lowest shortening rates (Fig. 6).
Outcrop-Scale Strain Analysis
Structural mapping was performed at Sweeney Road, near Lompoc, California (Fig. 1A). The investigated section at Sweeney Road is folded into an anticline to the north and a syncline to the south. The anticline-syncline pair is folded into concentric and chevron-type folds with fold angles of 68° for the anticline and 98° for the syncline, with steeply north-dipping axial planes (Figs. 7 and 8).
The relative distribution of diatomite to chert/porcelanite changes progressively throughout the Sweeney Road section, with pure diatomite in the Sisquoc Formation to chert/porcelanite-dominated intervals in the upper Monterey Formation (Figs. 3, 7, and 8). Three zones of distinct contrasting mechanical properties were identified. Zone 1 is in the lower Sisquoc Formation, and it is mechanically homogeneous. The main lithology is diatomite. Zone 2 is the initial diagenetic transition zone in the upper Monterey Formation, and it is composed of alternating meter-scale intervals dominated by either diatomite or chert/porcelanite. Zone 3 exposes the thinly bedded upper Monterey Formation that is characterized by rhythmic interbedded chert/porcelanite and more porous and detritus-rich diatomaceous sediment (Fig. 8). The three identified zones are consistent with the three siliceous diagenetic stages explained in Figure 3. The most prominent outcrop-scale structures were investigated at five locations, each with different mechanical and/or map-scale structural settings (Fig. 8, boxes A–E). The first location (Fig. 8, box A) was located on the north limb of the anticline in zone 1 and composed entirely of diatomite of low competence. No outcrop-scale structures were observed within this domain. The second location (Fig. 8, box B) was structurally located on the north limb of the anticline in a diagenetic transition zone with thickly interbedded diatomite-dominated to chert/porcelanite-dominated strata through the section (zone 2). The most prominent structures are Z-shaped folds with differing scales and wavelengths. Folds occur through the more competent beds that are dominated by cherts and porcelanites. The porous diatomite intervals are also displaced by the folds, but the folds vanish away from the folded chert/porcelanite-dominated package (Fig. 7; Fig. 8, box B). Fold wavelengths differ with layer thickness; thicker competent packages display longer fold wavelengths than the thinner competent packages (Fig. 8, boxes B and C). The third location (Fig. 8, box D) was structurally located on the south limb of the anticline. Lithologically, it consists of a thick package of thinly bedded chert/porcelanite-dominated lithologies toward the center of the anticline (zone 3) overlain by thinner packages of chert/porcelanite-dominated lithologies within a thicker interval of incompetent diatomite toward the center of the syncline (zone 2). The largest and most prominent outcrop-scale structure within this domain is a low-angle (relative to bedding) limb thrust fault that transects a more competent section of chert/porcelanite-dominated rocks and penetrates into the more incompetent section of diatomite-dominated rocks via a hanging-wall cutoff. This geometry produced a variety of lower-order folds and internal footwall buckle folds and a hanging-wall fault-bend fold (Fig. 8, box D). The structural position of the fifth zone was on the south limb of the syncline and inhabited zones 1–3. Similar to the first location, no major outcrop-scale structures were observed within this domain.
We present a deformation model based on analysis of the strain variations between the mechanically homogeneous diatomite of the Sisquoc Formation and the thinly and mixed bedded diatomite and chert/porcelanite of the upper Monterey Formation. Note, again, that although the diagenetic boundary between opal-A diatomite above and mixed opal-A and opal-CT lithologies below is not the formal distinction between the two formations, it does separate them in the study area for the most part. The model integrated observations made at map scale and outcrop scale and has implications for predicting structures in the subsurface.
Strain Differences between the Sisquoc Formation and Upper Monterey Formation
Strain analysis results show that measured shortening fluctuates up to 15.6% in the Lompoc–Santa Rosa fold belt and 12.1% in the Lompoc-Purisima anticline fold belt, respectively, over subregional-scale distances (Fig. 5), suggesting that the amount of measurable fold-related shortening directly relates to the predominant lithology and diagenetic state of the formation that was measured for structural data. Measurable shortening in the thin-bedded and more competent upper Monterey Formation was found to be twice as high as that in the overlying thick-bedded and less competent Sisquoc Formation (see shortening estimates and results table in Wirtz  and Supplemental Material).
Three explanations exist for these differences in measured strain between the formations: One possibility is that an unconformity exists between the Monterey Formation and the Sisquoc Formation, and the two formations experienced different strain histories. An uplift event during the late Miocene Rafaelan orogeny has been documented through localized erosional unconformities at the base of the Sisquoc Formation in the San Rafael Mountains ~40 km to the north (Dibblee, 1950, 1982) and in the Santa Maria Basin along many anticlines (McCrory et al., 1995). Although there is evidence for a depositional hiatus between the two formations in the west-central part of the Lompoc–Santa Rosa fold belt (Johns-Manville/Celite quarry; Barron and Ramirez, 1992), there is no evidence for an unconformity at other locations in the study area, including the Sweeney Road (Barron and Ramirez, 1992) and Lompoc Hills sections (Ramirez and Garrison, 1995) or in mapping of the study area (Dibblee, 1950). Furthermore, most widespread shortening occurred after the deposition of the Sisquoc Formation (Dibblee, 1950; Namson and Davis, 1990). Therefore, the contact between the Sisquoc Formation and the Monterey Formation is generally conformable across the study area, and the actual tectonic shortening history of both units is assumed to be identical.
A second possibility is that the two units are separate structural systems and are decoupled via a detachment fault. In this scenario, the Monterey Formation would have undergone deformation by folding, while the Sisquoc Formation would have undergone less deformation, with slip being consumed by a basal detachment fault instead of intraformational folding. However, this explanation is incompatible with the occurrence of map-scale fold axes that remain consistent across the contact between Monterey and Sisquoc Formations without significant changes in orientation of bedding strike and dip. Furthermore, folds remain harmonic and parallel across the two formations at the largest stratigraphic scale, just changing in amplitude and acuteness (Figs. 7, 8, and 9), and no regional detachment horizon between the Sisquoc and the Monterey Formations has been mapped by previous workers (Dibblee, 1950, 1993a, 1993b; Dibblee and Ehrenspeck, 1988a–1988e), or observed during fieldwork in this study.
A more likely, third explanation for the gross differences in measured strain between the formations relates to the different mechanical rock properties between strata at different siliceous diagenetic stages (Fig. 3). Our preferred deformation model—based on observations of the structural behavior of the different siliceous diagenetic stages—is as follows: In the study areas, diagenetic modification resulted in a thin-bedded, mechanically contrasting, and more competent upper Monterey Formation and a thick-bedded, mechanically homogeneous, highly porous, and less competent overlying Sisquoc Formation. Strain quantification at outcrop scale and microscale between different silica phases has already shown that the fold strain of competent chert/porcelanite intervals at outcrop scale is much higher than the fold strain of interbedded diatomites, and that the missing fold strain (simple shear) is accommodated by layer-parallel strain (pure shear) in the diatomite (Behl, 1992). Therefore, regional tectonic strain is accommodated via a different combination of strain mechanisms by units with different silica-phase rocks, so that their total shortening budgets still match, even if measurable fold strain is different. This deformation model accounts for the same amount of strain being experienced by the upper Monterey and Sisquoc Formations, but through different mechanisms. Strain is displayed by open to close folding and faulting in the brittle diagenetic rocks of the Monterey Formation but by horizontal compaction and gentle to open folding in the diatomaceous Sisquoc Formation (Fig. 9A). Therefore, total fold strain at subregional scales is best measured by restoring fold structures of the upper Monterey Formation because it is mechanically competent enough to respond to stress via volume-conservative folding and not horizontal compaction like the Sisquoc Formation. Representative sections for the Monterey Formation in the two study areas showed 21.1% overall shortening in the Lompoc–Santa Rosa fold belt and 16.8% overall shortening in the Lompoc-Purisima anticline fold belt. A structure contour map from a representative area in the Lompoc–Santa Rosa fold belt was generated using the cross sections created in LithoTect (Fig. 9B, top right). The map shows short-wavelength folds in areas with Monterey Formation surface exposure east of profile B–B′ (Fig. 9A). These folds vanish toward the west and transition up section into larger-wavelength folds documented in the Sisquoc Formation exposed at the surface (Fig. 9C). The structural style of tighter folding in the Monterey Formation should continue in the subsurface where covered by the Sisquoc Formation, as shown in schematic profile B–B′ (Fig. 9C).
The observations and interpretations made here show that significant fold-measured strain variation can occur due to differences in rock rheologies at formational scale. This is expressed laterally along the surface by either climbing up section into the Sisquoc Formation or dropping down section into the Monterey Formation (Figs. 9A and 9C). The large observable strain contrast is due to the extreme differences in competence and rheology of the primarily diatomaceous Sisquoc Formation and the chiefly cherty/porcelanitic Monterey Formation. The strain contrast effect on the construction of cross sections would be much smaller in nonsiliceous rocks with lesser contrasts in porosity and competency. This highlights the finding that variations in the competence of geologic units can be important components in the construction and assessment of structural models in different local sections of sedimentary basins.
Interpretation of Outcrop-Scale Structures
The most common outcrop-scale deformational structures in the upper Monterey Formations (aside from bed-confined fractures) are parasitic S- and Z-type folds along the limbs of the folded anticlines and synclines (Figs. 7 and 8), which reflect strata-parallel flexural shear. For example (looking east at Sweeney Road), parasitic folds with Z-type vergence occur on the north limb (Fig. 7; Fig. 8, boxes B and C), and lower-order parasitic S-type folds occur on the south limb of a map-scale anticline (Fig. 7; Fig. 8, box D). Locally, however, flexural shear is accommodated by faulting and bed-parallel slip. A particularly notable out-of-the-syncline thrust fault intersects the S-type folds and is the dominant counterclockwise shear structure on the south limb at box D (Fig. 8). It likely detaches from a competent section of dominantly interbedded chert/porcelanite and diatomite and then penetrates into an incompetent section of dominantly diatomite via a hanging-wall cutoff. This produced a domain of lower-order buckle folds in the footwall and a hanging-wall fault-bend fold (Fig. 8, box D), which developed here instead of parasitic S-type folds. Slip within these locations is along bedding-plane detachment faults out of the map-scale fold hinges that create high-strain regions expressed by small faults and buckling of the thinly bedded upper Monterey Formation instead of progressive folding. Both the parasitic folds and fault-related buckles are the result of decoupling mechanisms during flexural-slip deformation along bedding planes. The compatible shear direction of most parasitic folds and fault-related folds suggests that outcrop-scale folding and faulting were contemporaneous with map-scale folding. This would indicate that fold and fault strain at outcrop scale is not additive to the total strain at map scale and therefore would not have to be included in a calculation of total regional tectonic strain.
Outcrop-scale deformation is apparently limited to tight map-scale fold limbs where diatomite and chert/porcelanite occur in thin interbeds or alternating packages of different thickness (Figs. 7 and 8). For example, on the south limb of the open syncline at Sweeney Road, no larger outcrop-scale structures were observed (Fig. 8, box E). This location is representative of the majority of the exposures in the upper Monterey Formation throughout the Lompoc–Santa Rosa fold belt. The Sweeney Road section is located within the tightest folds across the entire fold belt in the upper Monterey Formation (Fig. 1A), and the northern anticlinal fold is much tighter than the open southern syncline.
Within the north limb of the anticline at Sweeney Road, parasitic folding is both harmonic and disharmonic (Fig. 7; Fig. 8, box B). We attribute the difference in behavior in the same location to result from layer differences in deformation mechanism (simple shear of the chert/porcelanite-dominated interbeds and pure shear of the diatomite-dominated packages), bedding thickness, and interbedded contrasting-competence beds. The differences in competency and porosity (and hence the ability to undergo pure shear compaction) between diatomite and chert/porcelanite are greater than those produced by any other known diagenetic transformation.
There are two situations that develop harmonic and disharmonic folding. First, under the influence of flexural-slip deformation along the bedding planes, thinly interbedded chert/porcelanite-dominated intervals can develop disharmonic folds above a bedding-plane detachment (Fig. 7). Slip on the detachment surface provides the fold shortening to the higher beds, and the fold axis terminates downward into the detachment. If no detachment surface is present, folding becomes harmonic without termination of the fold axes (Fig. 7). Second, if competent chert/porcelanite-dominated interbeds are separated by a sufficiently thick and incompetent diatomaceous mechanical layer, the fold vanishes, and the individual competent interbeds behave as if they are mechanically detached from each other and develop their own dominant wavelength without a fault separating the individual folds (Figs. 7 and 8). If competent chert/porcelanite-dominated interbeds are separated by a relatively thin incompetent mechanical layer, but in close proximity to each other relative to their own thickness, then the fold of the competent interbeds reaches the next package of competent interbeds and the folding remains harmonic (Fig. 8, box C). These observations suggest a relationship between the magnitude of folding in the deformed chert/porcelanite-dominated intervals and the thickness of intervening diatomite intervals, which depends on the extent to which local strain can be absorbed by the diatomite without failing and faulting occurring. As a consequence, very small amounts of buckling can be harmonic between individual beds separated by only centimeters, but as the amplitude of folds increases, incorporation of thicker stratigraphic units of porous diatomite would prevent folding from remaining harmonic. Therefore, in zone 2 (Fig. 8), folding within a thinly interbedded chert/porcelanite-dominated interval tends to be harmonic because of the thin diatomaceous interbeds, but folding becomes disharmonic as thick, intervening beds of diatomite allow subsequent packages of thinly bedded chert/porcelanite to be folded in an out-of-phase geometry (Fig. 7; Fig. 8, boxes B and C). Deformation of incompetent layers that undergo folding as a result of the folding of adjacent competent layers has been described as contact strain (Ramberg, 1962). In incompetent rocks with sufficient distance from the main fold, the folding becomes negligible, and simple shear translates into pure shear, and no contact or fold strain occurs. The documented deformation mechanisms (pure shear and simple shear) at outcrop scale suggest that the mechanical stratigraphy is the main controlling element for decoupling along and within the Sisquoc Formation to upper Monterey Formation diagenetic boundary.
The zones within incompetent diatomaceous packages that translate fold strain into volumetric strain (zones of contact strain into zones of no contact strain) detach the fold strain at outcrop scale and provide an explanation for the existence of the distinct deformational styles between the purely diatomaceous Sisquoc Formation and the thin-bedded opal-CT chert/porcelanite-dominated upper Monterey Formation at fold-and-thrust-belt scale without a separating detachment fault (Fig. 9). As shown at outcrop scale, the map-scale structural style progressively changes up section from tighter, subregional folding in the upper Monterey Formation to a zone of contact strain folding in the lower Sisquoc Formation to gentle regional folding with no contact strain in the upper Sisquoc Formation (Fig. 9).
The mix of deformation styles found across the three zones of the diagenetic transition can create complications and pitfalls to structural interpretation. For example, wells drilled through the tightly folded upper Monterey Formation are predicted to encounter and record inconsistent dip angles that reflect the deformation observed at outcrop scale (Fig. 9). Therefore, modeling of regional subsurface structure requires some caution in picking representative dip angles from dip-meter logs that transect tightly folded upper Monterey Formation strata. In gently folded sections, however, the dip meters are expected to be consistent and representative of the larger map-scale structures.
Several key conclusions arise from this strain analysis of the Monterey Formation and overlying Sisquoc Formation in the southern Santa Maria Basin:
Significant fold strain variation along strike can occur due to differences in rock rheologies related to silica phases at formational scale.
Mechanical stratigraphy (resulting from original sediment composition and degree of silica diagenesis) is the main controlling element for decoupling along the Sisquoc Formation to upper Monterey Formation diagenetic boundary.
Parasitic folding at outcrop scale is not pervasive and only occurs within tight map-scale anticline-syncline pairs. Its shear direction suggests that this deformation occurred coeval with the map-scale folding.
Thick diatomaceous intervals within stratigraphic sections can absorb and terminate the fold strain at outcrop scale by transferring fold strain into volumetric compaction.
Distinct deformational styles can coexist in close proximity without a fault detachment between the purely diatomaceous Sisquoc Formation and the thin-bedded chert/porcelanite-dominated upper Monterey Formation.
This research was supported by the corporate affiliates of the California State University–Long Beach Monterey and Related Sedimentary Rocks (MARS) project. The manuscript and the Master’s thesis from which it is derived greatly benefited from discussions with Thom Davis, Jay Namson, and Nate Onderdonk. We appreciate the valuable reviews by Alexandra Abrajevitch and Gregg Blake. We also thank the Santa Barbara Museum of Natural History for permission to use the D&E 1988 basemaps in Figure 1A.