Quartzose Petrofacies Research Articles

Provenance, diagenesis and reservoir quality of sandstone facies of the Maracangalha Formation, Recôncavo Basin – Northeastern Brazil

Sedimentary composition studies on siliciclastic reservoirs and their relationship with sedimentary provenance have proved to be useful in hydrocarbon exploration, enabling the prognosis of reservoir quality and diagenetic models. However, unconventional terrigenous reservoirs in the Maracangalha Formation, Upper Cretaceous from Recôncavo Basin, have not yet been evaluated regarding sedimentary provenance and its control over reservoir quality. This study aims to define the provenance of the Maracangalha Formation sandstones and its control on the diagenetic processes affecting the reservoir quality. Based on petrography analysis and X-Ray Fluorescence data, it was possible to identify a granite-gneissic terrain under wet climate as the primary source area for quartz-feldspathic sandstones. The granite-gneissic terrains with predominantly felsic minerals, within in the surrounding Precambrian Terrains, represent a major source area for the quartz-feldspathic sandstones found in the Maracangalha Formation. These sources are characterized by significant contributions from recycled sedimentary and metasedimentary rocks, leading to the occurrence of litho and quartzose petrofacies. The high chemical-textural sediment maturity and paleoweathering dispersion, under semi-umid to semi-arid climates, may be a result of paleoclimatic changes and the variable residence times of the sediments in the dispersion routes or temporary storage. The mineral composition and primary texture of sandstones show a clear influence on diagenetic evolution, particularly in terms of mechanical compaction inhibition, by early cementation, and the generation of secondary porosity due to the dissolution of framework grains and early cements. Therefore, understanding the sedimentary provenance, composition, and diagenesis is crucial for predicting the quality of the reservoir and the compartmentalization of the Maracangalha Formation and its analogues.

Interpreting detrital modes and geochemistry of sandstones from the late Paleozoic Tepuel-Genoa Basin: Paleogeographic implications (Patagonia, Argentina)

The provenance of sandstones deposited in the late Paleozoic Tepuel-Genoa Basin is analyzed in this paper. Five sections were sampled in Esquel, Sierra de Tepuel, Sierra de Tecka, El Molle, and Río Genoa areas for petrographic and geochemical studies. The sandstones in the Tepuel-Genoa Basin are dominated by feldspathic litharenites and litharenites, showing lithic fragments of volcanic and sedimentary rocks in the Valle Chico Formation and medium-to high-grade metamorphic rock clasts in the rest of the units.Detrital modes of seventy-five sandstones samples from the Valle Chico, Pampa de Tepuel, Mojón de Hierro, and Río Genoa formations were counted and analyzed. Seven modal components have discriminant value for identifying provenance areas (Qm, Qi, Lv, Lmm-h, Lm-Lp, Lm, Qpm). These modal components allow identification of three petrofacies: 1. Quartzose-lithic (Qm69Lv2Lm29), 2. Quartzose (Qm89Lv4Lm7) and 3. Volcanic-sedimentary (Qm60Lv38Lm1). The quartzose-lithic petrofacies is mainly composed of monocrystalline quartz, medium- and high-grade metamorphic clasts and polycrystalline quartz with cataclastic texture, this assemblage is interpreted as being derived from the crystalline rocks that form the Deseado Massif. The quartzose petrofacies is composed of monocrystalline quartz with scarce contributions of metamorphic clasts and traces of volcanic fragments; the provenance area is ascribed to sedimentary terrains, which most likely covered part of the Deseado Massif. The volcanic-sedimentary petrofacies is comprised of volcanic (acidic and intermediate rocks) and sedimentary (sandstone and mudstone) clasts, with discrete amounts of quartz grains with idiomorph shapes and embayments. This assemblage may correspond to material supply from the Devonian-Early Carboniferous accretionary complex developed in Chile or the unroofing of the western volcanic arc located in the central part of Patagonia. The validity of the three defined petrofacies was evaluated using Principal Component Analysis and triangular compositional diagrams; both methods show good separation and lack of overlap between the three petrofacies. Major (Si, Al, Fe, Na, K) and trace-REE elements (Zr, Th, Sc, Hf) were used to improve the petrographic information. The relation SiO2 against K2O/Na2O indicates that the Pampa de Tepuel and the Mojón de Hierro formations correspond to a passive margin, while the Valle Chico and Río Genoa formations represent different types of active continental margins. The Th/Sc and Zr/Sc ratios and the Th-Hf-Co distributions indicate that the sandstones of the Tepuel Group were formed from rocks compatibles with the average composition of the upper continental crust.

Provenance analysis of the Voirons Flysch (Gurnigel nappe, Haute-Savoie, France): stratigraphic and palaeogeographic implications

The Chablais Prealps (Haute-Savoie, France) represent a well-preserved accretionary wedge of the Western Alpine Tethys. They comprise a stack of sedimentary nappes related to palaeogeographic realms ranging from the Ultrahelvetic to the Southern Penninic. The provenance analysis is based on the Gazzi-Dickinson method and on QEMSCAN® for heavy-minerals. The Quartzose petrofacies is the most important of the two sources, and supplied three of the four formations of the Voirons Flysch. It is similar to the sources that fed the other flyschs from the Gurnigel nappe. It is characterised by a mature, quartz-rich assemblage and a heavy-mineral population dominated by apatite and the zircon–tourmaline–rutile mineral group. These observations suggest a Clastic wedge provenance. The Feldspathic petrofacies is derived from a feldspar-rich source associated with metamorphic clasts and a heavy-mineral population dominated by garnet. This provenance characterises only one formation of the Voirons Flysch, and is related to the axial belt provenance. This provenance analysis shows that the Middle Eocene to Early Oligocene Voirons Flysch was fed by two sources, in contrast to the other flyschs of the Gurnigel nappe, and further suggests that this flysch was not deposited in the Piemont Ocean but in the Valais domain. Based on the results and comparative provenance analysis with the other flyschs of the Gurnigel nappe, we propose a generic feeding model which involves the Sesia–Dent Blanche nappe, the sedimentary nappes incorporated in the accretionary prism, and probably the Brianconnais basement.

Interpreting provenance relations from sandstone detrital modes, southern Italy foreland region: Stratigraphic record of the Miocene tectonic evolution

The Southern Italy orogen preserves deep-marine turbitic clastic wedges that record the key tectonic events during structural growing over the Adria margin. The regional tectonic transition from thick-skinned Calabrian accretion and deformation (during Late Paleogene-to-middle Miocene) to thin-skinned Southern Apennines thrusting (since early-middle Miocene) is recorded by diverse contrasting petrofacies in uppermost Paleogene and Miocene sandstones of dominantly turbiditie deposits. Most of these sandstones reflect in time quartzolithic and quartzofeldspathic and arkosic petrofacies derived from growing and unroofing Calabrian terranes. Quartzose petrofacies mainly indicate provenance from cratonal African margin, during Langhian, and/or from lowland internal domains before its deformation, during the Cretaceous to Paleogene. Volcaniclastic sandstone petrofacies also testify active volcanism during the Late Paleogene to early Miocene. These variable compositions indicate strong palaeogeographic constraints for the central-western Mediterranean, during the Neogene, reflecting provenance relationships from differentiate source rocks. These include interplays between ophiolitiferous, uplifted continental shallow to deep crust terranes, volcanic and sedimentary (mostly carbonate) source rocks. Sandstone detrital modes provide an example of the close relations between clastic compositions and growing orogen in southern Italy and may contribute to unravel the geodynamic history of other major orogens.

Recycling detrital zircons: A case study from the Cretaceous Bisbee Group of southern Arizona

Statistically indistinguishable U-Pb age spectra for detrital zircon populations from the quartzose petrofacies of the fluvial Lower Cretaceous Bisbee Group (southeastern Arizona) and from quartzose eolianites of the Middle to Upper Jurassic San Rafael Group (eastern Colorado Plateau) support the inference that quartzose detritus was reworked into the Bisbee basin of the border rift belt from older eolianites exposed along the uplifted rift shoulder to the north. The comparative data indicate that age spectra of detrital zircons can be reproduced faithfully during sediment recycling, and the possibility of zircon recycling should be considered for many provenance interpretations based on U-Pb ages of detrital zircons. Analogous relations between U-Pb age spectra of detrital zircons in the Lower Cretaceous basal McCoy Mountains Formation (California) and in Lower to Middle Jurassic eolianites of the western Colorado Plateau to the north suggest similar reworking of detritus from sedimentary cover of the rift shoulder into the keel of the rift belt farther west.

Integrating Sandstone Petrology and Nonmarine Sequence Stratigraphy: Application to the Late Cretaceous Fluvial Systems of Southwestern Utah, U.S.A.

ABSTRACT Petrographic and dispersal data are essential to correct interpretation of mechanisms that create continental sequence-stratigraphic architecture. A case study from southern Utah demonstrates that Upper Cretaceous (upper Santonian-Campanian) alluvial successions in the southernmost part of the Cordilleran foreland basin were deposited by fluvial systems of contrasting drainage directions and provenance, and suggests that different mechanisms governed their sequence architecture. Most of the rivers flowed northeast, subparallel to the basin foredeep. Less common fluvial systems flowed to the east-southeast. The fluvial sandstones fall naturally into four petrofacies: (1) quartzofeldspatholithic (mean Qt61F19L20); (2) feldspatholithic (Qt29F19L52); (3) quartzolithic (Qt75F6L20); and (4) quartzose (Qt99F1L1). Petrofacies 1 and 2 were derived from mixed supracrustal and basement sources to the southwest and south, respectively, whereas petrofacies 3 and 4 were derived from uplifted thrust sheets of the Sevier orogenic belt to the southwest and west, respectively. Only the east-southeast-flowing rivers transported the quartzose petrofacies. The fluvial strata, which include the uppermost Straight Cliffs, Wahweap, and Kaiparowits formations, form two large-scale stratigraphic successions typically interpreted as continental stratigraphic sequences hundreds of meters thick. Each succession begins with an amalgamated braided-fluvial deposit, grades to mudstone-rich strata with low sandstone-body connectivity, and culminates in highly connected sandstone bodies with multistory stacking. The basal amalgamated deposits of each succession are architecturally similar, but their compositional and dispersal characteristics are different. Quartzofeldspatholithic, quartzolithic, and quartzose sandstones above the lower base-level shift are variable, but generally similar in compositional and dispersal characteristics to both underlying and overlying strata, a phenomenon termed here congruence. In contrast, quartzose amalgamated fluvial sandstone above the upper base-level shift differs sharply in composition and dispersal direction from underlying and overlying lithic-rich strata. The foredeep axis controlled the progradation direction of the congruent shift, which was likely driven by climatically induced sediment influx, a eustatic fall, or both. In the case of the incongruent shift, increased sediment supply permitted the rivers to cross the foredeep. Temporal association of the upper amalgamated deposit with active structures in the thrust belt and foreland basin indicates that syntectonic thrust uplift, not isostatic uplift or climate, caused the influx of quartz.

Tectonic setting and sandstone petrofacies of the Bisbee basin(USA–Mexico)

The Late Jurassic to mid-Cretaceous Bisbee basin spanning the USA–Mexico border was part of the Border rift system, which extended into the continental block from the Gulf of Mexico in response to Cordilleran slab rollback. Rift initiation was marked by eruption of a bimodal mid-Jurassic volcanic assemblage succeeding Early to Middle Jurassic arc volcanism. The core of the Bisbee basin is delineated by Upper Jurassic marine strata of limited extent, and by more widespread syntectonic conglomerate of Late Jurassic to Early Cretaceous age derived from intrabasinal fault blocks of synrift paleotopography. Subsequent thermotectonic subsidence of the rift belt induced Early Cretaceous marine and nonmarine sedimentation spreading from the basin core to its flanks, with peak Aptian–Albian transgression marked by deposition of platformal limestone. Late Cretaceous to Paleogene Laramide deformation disrupted the Bisbee deposystems. Bisbee sandstones include arkosic petrofacies derived from intrabasinal fault blocks, lithic (volcaniclastic) petrofacies derived principally from the coeval Alisitos arc to the southwest, subquartzose petrofacies derived mainly from the Mogollon paleohighland forming the northern rift shoulder, and quartzose petrofacies also derived from the rift shoulder. The joint association of disparate petrofacies reflects the unusual geotectonic setting of the Border rift belt.

Provenance of early Palaeozoic sandstones, southeastern Australia, Part 2: cratonic to arc switching

Abstract Early Palaeozoic strata of the northeastern Lachlan Fold Belt, Australia, consist of an Early Ordovician sequence (Adaminaby Group) of quartz-rich turbidites and chert, and the conformably overlying Late Ordovician–?Early Silurian Coomber Formation, a mafic volcaniclastic sequence of lithic sandstone and mudstone with mafic volcanic and intrusive rocks. Analysis of detrital modes from the Adaminaby Group indicate a uniform quartzose petrofacies (Q 91 F 2 L 7 ) composed of deep-water quartz and sublithic sandstones derived from the west. By contrast, the lithic sandstone of the Coomber Formation represents a volcanolithic petrofacies (Q 1 F 34 L 65 ) of mainly intermediate volcanic detritus with a tendency to become more silicic up-section. These sandstones are considered to be derived from a calc-alkaline andesitic–basaltic volcanic centre in the eastern Lachlan Fold Belt. The change from a quartz-rich petrofacies to a volcanolithic petrofacies was sharp with no evidence of mixing. This change primarily reflects a rapid outpouring of mafic volcanic material in the Late Ordovician, together with an approximately concurrent eustatic rise in sea level.

Source Versus Depositional Controls on Sandstone Composition in a Foreland Basin: The El Imperial Formation (Mid Carboniferous-Lower Permian), San Rafael Basin, Western Argentina

ABSTRACT The El Imperial Formation (mid-Carboniferous-Lower Permian) constitutes a progradational sandstone-rich succession deposited in the San Rafael foreland basin of western Argentina. Four facies associations have been identified: a basal glacial marine association, a shallow marine association, a deltaic association, and an uppermost fluvial association. Sand-prone deposits in the deltaic association are represented by prodelta and delta-front shales and subordinate fine sandstones (Facies A), deltaic platform, wave-reworked channel mouth-bar sandstones (Facies B), and fluvial-dominated distributary channel sandstones (Facies C). Analysis of framework grains of sandstone samples from Facies B and C shows two distinct mineral assemblages or petrofacies. The quartzose petrofacies is characterized by high contents of quartz and low percentages of feldspar and lithic grains. The quartzolithic petrofacies shows an increase in labile components, in particular lithic fragments, and a concomitant decrease in quartz. The quartzolithic petrofacies shows a source signature. Average detrital modes of sandstones from this petrofacies are similar to those from overlying fluvial sandstones. All wave-reworked, channel mouth-bar sandstones (Facies B) correspond compositionally to the quartzose petrofacies, whereas detrital modes from the distributary-channel sandstones (Facies C) fall into the quartzolithic petrofacies. This correspondence between depositional environment and petrofacies suggests a strong depositional influence on composition (depositional signature). Abrasion (mechanical breakdown) by wave action in shallow marine environments accounts for the quartz-rich nature and paucity of labile grains in the quartzose petrofacies.

Compositional variation in depositional sequences of the upper part of the Kazusa Group, a middle Pleistocene forearc basin fill in the Boso Peninsula, Japan

Shelf to slope successions of the upper part of the Kazusa Group, as much as 700 m thick, constitute a northeasterly prograded clastic wedge of middle Pleistocene age and consist of six depositional sequences. Fundamental component building blocks of depositional sequences (i.e. lowstand, transgressive and highstand systems tract sediments) are characterized by unique combinations of depositional processes and petrofacies. Transgressive systems tract sediments are characterized by a high frequency of volcanic ash layers and volcanogenic petrofacies, indicating a substantial contribution of pyroclastic material to marine environments. This corresponds to the storage of coarse-grained, terrigenous clastic sediments at a river mouth area during transgressions. Highstand systems tract sediments are characterized by a lower frequency of volcanic ash layers and more quartzose petrofacies, and may have been affected by abrasion and destruction of unstable lithic fragments by nearshore dynamic processes, such as reworking by waves, tides and longshore currents, in response to active progradation of a coastal depositional system. In general, progradation of coastal depositional systems is associated with active supply of coarse-grained, terrigenous sediments by a fluvial system. The increase in supply of coarse-grained terrigenous sediments reduces the frequency of volcanic ash layers in the upper part of the highstand systems tract sediments. Lowstand systems tract sediments show more variable petrofacies than that of highstand systems tract sediments and may have been affected much more by the direct fluvial supply of coarse-grained, terrigenous sediments, which were not subjected to abrasion and destruction of unstable lithic fragments by nearshore dynamic processes. These results demonstrate that the development of the upper part of the Kazusa Group can best be interpreted in terms of the interaction between marine and fluvial processes, and fluctuation in the relative rate of coarse-grained, terrigenous sediment supply. The frequency of volcanic ash layers and the relative contents of unstable volcanic rock fragments in marine sediments may be a sensitive indicator of the dynamic processes and may provide an additional opportunity for characterizing systems tracts within a depositional sequence in a volcanic arc setting.

Sandstone Detrital Modes in the Paleogene Liguride Complex, Accretionary Wedge of the Southern Apennines (Italy)

ABSTRACT The Liguride Complex is an ophiolite-bearing unit related to westward subduction of the lithosphere of the Neo-Tethys Ocean beneath the European margin. The complex includes a melange (the Calabro-Lucanian Flysch Unit) containing broken formations of ophiolites and related pelagic and clastic sediments (radiolarian chert, limestone, black shale, and quartzarenite), olistoliths or tectonic slices of continental crest (gneiss, granite, and amphibolite), Middle Eocene to Upper Oligocene siliciclastics, and a less deformed turbidite unit (the Saraceno Formation) that caps the succession. Detrital modes of Upper Cretaceous to Lower Miocene sandstones from the Liguride Complex accretionary wedge are grouped into four distinct petrofacies that reflect the geodynamic evolution of Southern Italy. An Upper Cretaceous to Middle Eocene quartzose petrofacies (Q90F9L1) is composed of deep-water quartzarenitic and subarkosic sandstones that appear to have undergone long-distance transport by turbidity currents from the European or African foreland. A Middle Eocene to Upper Oligocene quartzofeldspathic petrofacies (Q62F21L17) was produced by mixing of sediment from ophiolitic and pelagic sedimentary sources with volcanic debris. An Upper Oligocene volcanolithic petrofacies (Q16F24L6054F10L36) is present in the turbidites of the Saraceno Formation. The accretionary-wedge sandstones of the Liguride Complex may record long-distance longitudinal transport for hundreds of kilometers from distal sources (preexisting forelands and volcanic arc) and short-distance transverse transport from local sources (accreted oceanic terranes and collisional-belt terranes), In accretionary-wedge sandstone assemblages, sediments from local and distal sources may be mixed or may record a consistent evolution from one type of transport to the other.

Sandstone Detrital Modes, Central Utah Foreland Region: Stratigraphic Record of Cretaceous-Paleogene Tectonic Evolution

ABSTRACT Within a sedimentary basin, the analysis of sandstone petrofacies defined by detrital modes of framework grains is a powerful means to detect areal or temporal changes in provenance related to evolving paleotectonics and paleogeography. In the central Utah foreland, the regional tectonic transition from thin-skinned Sevier thrusting during the Late Cretaceous to deep-seated Laramide deformation during the Paleogene is recorded by four contrasting petrofacies in uppermost Cretaceous and Paleogene sandstones of fluvial and deltaic facies. Coeval subaqueous environments were marine during the Cretaceous and lacustrine during the Paleogene. The Cretaceous quartzose petrofacies (Qm93, F1, Lt6) of mainly Campanian age was derived from sedime tary sources in the active Sevier thrust belt, and transported into a broad adjacent foreland basin by transverse paleocurrents. The heterogeneous Cretaceous quartzolithic petrofacies (Qm40-70, F5-20, Lt25-40) of uppermost Campanian age was derived from the Sevier orogenic belt during the waning phases of Sevier tectonism, and was distributed by longitudinal paleocurrents within a foreland basin whose floor had begun to be deformed by incipient Laramide tectonism. The Paleogene feldspathic petrofacies (Qm50, F35, Lt15) in Paleocene and Eocene strata is composed of arkosic detritus that was derived principally from basement rocks then exposed in the extensive San Luis uplift of Laramide age in south-central Colorado. The feldspathic sand was transported to the Laramide Uinta Basin and vicinity in Utah by a trunk river which flowed generally northwestward between monoclinal Laramide uplifts of the Colorado Plateau. The Paleogene quartzolithic petrofacies (Qm75, F2, Lt23) of dominantly Eocene age was derived mainly from sedimentary sources in the Laramide Uinta uplift to the north, but also in part from similar sources in the dormant Sevier thrust belt to the west. In the Rocky Mountain and intermountain regions, more widespread and systematic use of petrofacies analysis, in combination with standard paleocurrent and lithofacies analyses, could lead to better definition of the timing of Laramide uplift and improved understandi g of the evolution of complex Laramide paleogeography.

Fluvial Systems of Upper Cretaceous Mesaverde Group and Paleocene North Horn Formation, Central Utah: Record of Transition from Thin-Skinned Deformation in Foreland Region: ABSTRACT

Nonmarine strata of the upper part of the Mesaverde Group and North Horn Formation exposed between the Wasatch Plateau and the Green River in central Utah record a late Campanian tectonic transition from thrust-belt deformation to basement-cored uplift. Mesaverde Group sediments were deposited by synorogenic braided and meandering rivers. During most of Campanian time, sediment transport was east and northeast away from the thrust belt across a fluvial coastal plain. Subsequent development of the San Rafael swell, a basement uplift, between western and eastern localities caused erosional thinning of the section. Sandstones within the upper part of the Mesaverde Group form two distinct compositional suites, a lower quartzose petrofacies and an upper lithic petrofacies. Lithic grain populations of the upper petrofacies are dominated by sedimentary rock fragments on the west and volcanic rock fragments on the east. Sedimentary lithic grains were derived from the thrust belt, whereas volcanic lithic grains were derived from a volcanic terrane to the southwest. Tributary streams carrying quartzose detritus from the thrust belt entered a northeast-flowing trunk system and caused a basinward dilution of volcanic detritus. Disappearance of volcanic grains and local changes in paleocurrent directions in latest Campanian time reflect initial growth of the San Rafael swell and development of an intermon ane trunk-tributary fluvial system. Depositional onlap across the Mesaverde Group by the post-tectonic North Horn Formation indicates a minimum late Paleocene age for uplift of the San Rafael swell. End_of_Article - Last_Page 854------------

Late Cretaceous Fluvial Systems and Inferred Tectonic History, Central Utah: ABSTRACT

Upper Campanian nonmarine sedimentary rocks exposed between the Wasatch Plateau and the Green River in central Utah record a tectonic End_Page 1346------------------------------ transition from thin-skinned deformation in the thrust belt to basement-cored uplift in the foreland region. Thick sections of the Mesaverde Group in the Wasatch Plateau on the west and the Book Cliffs on the east are separated by the San Rafael swell, a basement uplift across which the group is erosionally thinned. Strata in the west (Castlegate Sandstone and Price River Formation) were deposited by east to northeast-flowing braided rivers. Time-equivalent eastern sections comprise a lower sequence of mixed braided fluvial deposits (Castlegate Sandstone and Bluecastle Tongue of Castlegate, coastal swamp and meander-belt deposits (Neslen Formation), and nearshore marine deposits (Buck Tongue of Mancos Shale and Sego Sandstone), and an upper sequence that coarsens upward from meander-b lt deposits (Farrer Formation) into pebbly braided river deposits (Tuscher Formation). Paleocurrent data indicate that rivers of the lower sequence flowed east, while those of the upper sequence flowed northeast. Figure Sandstones within the section consist of two distinct compositional suites, a lower quartzose petrofacies and an upper lithic petrofacies. The compositional boundary occurs at the top of the Bluecastle Tongue and can be correlated across the San Rafael swell. The quartzose suite contains mostly compositional quartzarenites and sublitharenites; the lithic suite is composed of litharenites and feldspathic litharenites. Lithic grain populations of the upper petrofacies are dominated by sedimentary fragments in sections of the Wasatch Plateau and volcanic fragments in sections near the Green River. The sedimentary lithic grains were transported generally eastward from miogeoclinal strata uplifted within the thrust belt. The volcanic lithic grains of the Farrer and Tuscher Formations were erived from more distal arc sources to the southwest, and transported through the thrust belt somewhere west of the Kaiparowits region, where time-equivalent sedimentary rocks are also rich in volcanic lithic fragments. Disappearance of volcanic lithics and appearance of pebbles at the top of the Tuscher Formation is interpreted to reflect a latest Campanian reorganization of drainage patterns that marked initial growth of the San Rafael swell and similar basement uplifts to the south of the swell. Contemporaneous fluvial systems that deposited the uppermost part of the Price River Formation in the Wasatch Plateau were apparently unaffected by the uplift and continued to flow northeast. Depositional patterns thus indicate that initial growth of the San Rafael swell was probably concurren with late deformation in the thrust belt. Depositional onlap across the Mesaverde Group by a largely post-tectonic assemblage of fluvial and lacustrine strata (North Horn Formation) indicates a minimum late Paleocene age for growth of the San Rafael swell and deformation within the thrust belt. End_of_Article - Last_Page 1347------------