Macalester College Geology Department

Authigenic Mineralization and Geochemical Taphonomy of Vertebrate Microfossils from the Upper Cretaceous Judith River Formation of Montana

Fossil bone is a unique medium in the study of early diagenesis because bone tissue becomes enriched in rare earth elements and includes void space that can readily accommodate authigenic mineralization. Previous studies of bone diagenesis, however, have either focused on vertebrate remains from an archaeological perspective, or have been limited to diagenesis within one paleoenvioronment, resulting in a gap in understanding of early diagenetic and taphonomic processes acting on vertebrate material across environments. The objective of this study was to compare vertebrate material from marine and terrestrial localities in the Upper Cretaceous Judith River Formation, with an emphasis on authigenic cementation and rare earth element concentration and fractionation patterns, with the goal of clarifying the relationships between bone mineral, trace elements, authigenic minerals, and water in the enclosing sediment during diagenesis.

The richly fossiliferous Campanian Judith River Formation (JRF) is widely exposed across much of north-central Montana, and is composed of coastal lowland and marine lithofacies. The ~180 m thick unit comprises the distal reaches of an eastward-thinning clastic tongue that accumulated on the western margin of the Cretaceous Western Interior Seaway. The JRF is a heterolithic composite of silty claystones, siltstones, and fine- to medium-grained sandstones of fluvial, tidal, and shallow marine origin. Authigenic cements and rare earth element (REE) signatures in ~110 fossil bones from two terrestrial sites and one marine site were analyzed using LA-ICP-MS, petrographic microscopy, and SEM-EDS to explore and compare preservational signatures.

Results suggest that rare earth elements and authigenic cements provide site-specific signatures in fossils bones, characterize the chemistry of pore-waters during diagenesis, and allow determination of the degree of mixing of a fossil assemblage. Fossil material preserved in shallow marine sandstones displays extensive phosphatization and pyritization, and is interpreted to be highly mixed (high variance in REE fractionation patterns). Fossil material from one terrestrial locality has a paucity of authigenic cements, and is consistently MREE enriched (low variance). The other terrestrial locality preserves fossil material with a more diverse suite of authigenic minerals including calcite, Ba/Sr sulfates, pyrite, and ankerite. Fossils are MREE/ HREE enriched, and also are not interpreted to have undergone significant pre-burial mixing.

Differences in authigenic cementation patterns and REE signatures are interpreted to represent a corresponding difference in physical and chemical conditions at and below the sediment-water interface. The authigenic cements and REE signatures indicate contrasting geochemical conditions, suggesting that they record different stages of fossilization, with rare earth elements recording the signature of the environment of deposition and authigenic cements indicative of the burial environment. In addition to characterizing the environment of fossilization, REE and authigenic cements used together may provide information about the formative history (i.e. degree of exhumation, reworking) of a fossil assemblage. Investigation of the relationships between preservation of vertebrate fossil material, in terms of geochemical taphonomy and authigenic mineralization, and preservational environment provides valuable information regarding geochemisty, diagenesis, and taphonomy in the Campanian Judith River Formation of Montana.

An Experimental and Field-Based Approach to the Taphonomy of Microvertebrate Assemblages: A Case Study in the Judith River Formation of North-Central Montana

Microvertebrate assemblages, or “microsites,” are fossilized concentrations of small physico-chemically resistant vertebrate elements, including small bones and teeth from mammals, crocodiles, turtles, fish, and dinosaurs. These assemblages are generally preserved in two distinct depositional settings: 1) low-energy pond/lake environments and 2) high-energy channel facies. Many previous works have focused on the origins of channel-hosted concentrations. Several of these studies examined the behavior of bone in an active flow, and most concluded that bone material has great dispersal potential in water. Other studies focused on the characterization of fossil microvertebrate assemblages and generally discount the possibility of using these sites for paleobiological reconstruction, claiming that the bones in these assemblages have been transported great distances from their original habitats. However, previous studies generally fail to provide a mechanism of bone concentration in a fluvial environment. How does bone material concentrate in an active channel if its predicted behavior in water is dispersal?

This study takes a two-fold approach to understanding the origins of microvertebrate assemblages by 1) comparing sites found in both lacustrine and channel deposits and 2) conducting flume experiments in which an active flow is directed through a bed of sediment with a known concentration of dispersed bone material. Lacustrine and fluvial assemblages can be readily distinguished based on sedimentary context and the overall richness, or abundance, of bone material. Moreover, taphonomic features, such as the degree of surface alteration and trends in size sorting, differentiate the two site types. Bone material from the low-energy lacustrine assemblages (Type I) is dispersed as a background concentration (<1% bone by volume) throughout the fine-grained mudstone deposits, while bone material in channel-hosted assemblages (Type II) is concentrated in discrete lags (up to 3% bone by volume) localized at the base of fine-grained fluvial deposits. Microvertebrate elements from both types of sites exhibit a range of rounding, fragmentation, alteration, size, and shape. However, bone material from Type II assemblages shows evidence of greater surface alteration (discoloration, weathering) and is better sorted compared to the bone material from Type I assemblages.

Flume experiments were conducted to better constrain potential origins of microvertebrate assemblages. An active flow was directed through beds of fine-grained sand and dispersed bone material with three different initial background bone concentrations (0.1%, 1%, and 10% bone by volume). In all three experiments, bone material became less evenly dispersed through time as local accumulations developed. Fine bone material (0.5-1 mm) was preferentially winnowed to the distal portions of the flume, while the coarsest fraction of bone material (2-4 mm) was transported minimally and generally remained in the proximal portion of the flume as lag deposits.

The potential for natural channels to interact with Type I (low-energy) assemblages and the characteristics of Type I and Type II (high-energy) assemblages indicate that Type II assemblages could readily be generated by reworking preexisting Type I deposits. Experimental results further demonstrate that erosional flow processes can generate localized concentrations of bone material from a background of widely dispersed material. Taken together, these findings indicate that microvertebrate assemblages found in ancient channel scours could plausibly be sourced locally from preexisting concentrations. In turn, this study has important paleoecological implications, specifically in relation to the spatial fidelity and temporal resolution of microvertebrate assemblages.

A Comparative Investigation of Diagenesis in Fossil Teeth and Fish Scales from the Upper Cretaceous Two Medicine and Judith River Formations of Montana

The Campanian Two Medicine (TMF) and Judith River (JRF) Formations of Montana preserve coeval deposits of the semi-arid alluvial uplands (TMF) and the coastal plain lowlands (JRF). Previous work has documented the abundance and quality of vertebrate fossils in both formations. In this study I explore the diagenesis of dinosaur teeth, crocodile teeth, and gar scales using REE content and authigenic cements. The fossils under investigation were collected from several well-documented sites of fluvial and lacustrine origin.

Authigenic cements filling cracks and dentine tubules were examined in 23 theropod teeth, 19 hadrosaur teeth, 6 crocodile teeth, 4 ceratopsian, ankylosaur and myledaphus teeth, and 8 gar scales using polarized light microscopy and SEM-EDS. Fills include calcite, sulfates, sulfides, bornite, authigenic and detrital clays, iron oxides, and clastic particles. Patterns of authigenic cementation serve to describe the general process of permineralization, and also characterize pore water chemistry in the burial environment.

Rare earth element (REE) concentrations were determined for the same sample of teeth and scales using LA-ICP-MS. TMF teeth have higher REE concentrations than JRF teeth and are enriched in middle REE. JRF teeth are enriched in HREE. TMF teeth also tend to show more variability in REE concentrations. These REE patterns suggest that (1) TMF teeth likely spent the early stages of diagenesis in or upon alluvial soils, (2) TMF teeth experienced greater small-scale variability in the factors controlling REE uptake, and (3) JRF teeth likely spent early diagenesis in stagnant aqueous environments. These findings are consistent with independent sedimentologic and taphonomic data, and are also in line with previous studies of other workers that focused on REE patterns in TMF and JRF bone.

Finally, the overall REE patterns in enamel and dentine are similar within a given element, but normalized total REE concentrations vary in these tissues from relatively low (enamel) to relatively high (dentine). This pattern is consistent with porosity and crystallite density contrasts in enamel and dentine, and illustrates the importance of these factors in diagenetic processes.

Revisiting the Magnetostratigraphy of the Upper Cretaceous Berivotra and Maevarano Formations, Northwestern Madagascar

The Maevarano and Berivotra Formations of the Mahajunga Basin in northwestern Madagascar yield a diverse assemblage of paleobiologically and paleobiogeographically significant vertebrate fossils. In an effort to correlate these units to the geomagnetic polarity time scale, oriented rock samples were collected from 28 sites. Magnetic remanence was detected with a Superconducting Rock Magnetometer (SRM) at the University of Minnesota’s Institute for Rock Magnetism, employing both alternating field (AF) and thermal demagnetization techniques. Although signals were weak, analysis thus far corroborates work done by Michelle Casey in 2003. Both studies correlated the Berivotra Formation and the uppermost Maevarano Formation (more tentatively) to Chrons 29R and 30N (late Maastrichtian). Additionally, trends were observed that might offer insight into future study: 1) Fidelity of magnetic signal seems to be distinctly tied to Lithologic character, and 2) There may be a reversed polarity or excursion recorded in Berivotra sediments. Based on this study, it is recommended that further work be conducted on specific units that may have more interpretable signals (specifically the Berivotra Formation and the Miadana Member of the Maevarano Formation). It is also suggested that extensive sampling be done in the interval 15-20 meters below the Berivotra Formation – Betsiboka limestone contact to test the hypothesis that a reversal or excursion is present.

Authigenic cements and Rare Earth Element Concentration in Fossil Bones from the Upper Cretaceous Two Medicine Formation, Northwestern Montana

The Upper Cretaceous (Campanian) Two Medicine Formation of northwestern Montana preserves abundant vertebrate fossils. An understanding of the changes that take place within a given bone as it becomes a fossil is essential to an informed interpretation of that fossil material and its associated depositional and diagenetic system. I investigated authigenic cements and rare earth element (REE) concentrations in fossil bones from a variety of terrestrial depositional environments, including high-energy fluvial deposits (sandstones) and low-energy floodplain and coastal deposits (mudstones). I explored preservational patterns in a paleoenvironmental context by comparing trends in authigenic cementation and REE concentration across differing lithologies, between similar depositional environments, and between individual fossil bones within a site. Authigenic cements were investigated using polarized light microscopy and scanning electron microscopy (SEM). REE concentrations in fossil bone tissue were examined using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS).

The nature and relative abundance of authigenic cements vary among sites. This suggests that authigenic cements track environments, and that there exists a correlation between depositional environment and authigenic mineralization. REE concentrations tended to display distinctly different fractionation patterns by site, and thus tracked depositional settings. In general, fossil bones from sandstones show the most variation in both authigenic cements and REE concentration, and are typified by HREE enrichment and abundant calcite and iron oxide void fill. Fossil bones from a coastal mudstone locality exhibit the least variability in preservational state, and are characterized by presence of pyrite, replacement of bone tissue by calcite, and very low levels of REE fractionation. Fossil bones from upland low-energy sites display intermediate variability and are characterized by MREE enrichment, and the presence of chlorite and detrital clay. Additionally, certain authigenic minerals and REE fractionation patterns are indicative of specific Eh/pH conditions. Overall, the data collected during the course of this multifaceted study allow for characterization of local conditions during early diagenesis, and reveal novel insights into the regional microtaphonomy of the Two Medicine system.

The Valley of Ten Thousand Smokes in Katmai National Park, Alaska as a Mars Analogue

Recent NASA missions to Mars have returned increasingly detailed surface imagery, motivating the effort to find and study analogue features on Earth to understand processes on Mars. The 1912 eruption of Novarupta in Katmai National Park created a barren volcanic landscape, the Valley of Ten Thousand Smokes (VTTS), which remains devoid of vegetation today. The precisely dated VTTS, created by pyroclastic volcanism, contains many geomorphic features similar to Mars and is an excellent test case for studying life in an inhospitable environment. The landscape would be beneficial for planetary science field studies, remote sensing and rover testing.

Geochemical analysis and characterization of paleosols from the Upper Cretaceous Masorobe Member of the Maevarano Formation, Mahajunga Basin, northwestern Madagascar

Paleosols, or ancient soils, are excellent indicators of climate because they tend to maintain much of their formative chemical and biological signatures. They have been identified and used for paleoenvironmental reconstruction in localities worldwide and in rocks dating as far back as the Precambrian. Upper Cretaceous terrestrial strata of the Maevarano Formation of the Mahajunga Basin in northwestern Madagascar preserve numerous exquisite paleosol profiles. Thirty-six well-preserved paleosol units were recently identified in the Masorobe Member, which comprises the lower 80 m of the Maevarano Formation, and six of the paleosols were described in detail and sampled at 10-cm increments. The Masorobe paleosols are alluvial in origin, compound, and generally red in color. They tend to consist of clay-rich silty sandstone. They are also characterized by pervasive pedogenic features such as color banding, strong vertical root traces, drab root halos, and slickensides. Pedogenic carbonate, in the form of carbonate nodules and carbonate-encrusted rhizoliths, is also common. The geochemistry of the six well-sampled paleosol units was analyzed using X-ray fluorescence spectrometry to assist in identifying diagnostic horizons indicative of organismal nutrient intake (A-horizons) and the illuviation of minerals and elements (B-horizons). Horizons were then used to classify the paleosols units from the Masorobe Member using a paleosol classification scheme designed specifically for fossil soils. Lithologic attributes suggest that these paleosols formed under alkaline conditions in a semiarid climate. Molecular weathering ratios, which represent the degree of hydrolysis and leaching, support a general interpretation of minimal to moderate weathering and leaching. Subtle differences developed among he Masorobe paleosols may possibly represent climatic variations that transpired during deposition of the 80-m-thick unit. Alternatively, the minor variations documented among the Masorobe paleosols may be due to (1) local variations in formative environment (topography and/or position on the floodplain), or (2) complications involving paleosol classification. Overall, the Masorobe Member paleosols provide key insights into paleoclimate of Madagascar.

Geochemical Charaterization and Discrimination of Bentonites in the Upper Cretaceous Two Medicine Formation, Northwestern Montana

Volcaniclastic terrestrial sediments of the Upper Cretaceous Two Medicine Formation of northwestern Montana preserve several discrete bentonite beds. These beds hold significant potential for geochemical fingerprinting and high-resolution correlation. This study focuses on three bentonites within the formation, and employs geochemistry to distinguish the "Seven Mile Hill bentonite" from the "Bed 2 bentonite" both found near Choteau, Montana, and the "Hadro-Hill bentonite" on the Two Medicine River. Major and trace element concentration were determined using X-ray fluorescence spectroscopy. The bentonite units successfully discriminated using bivariate plots and ternary diagrams utilized by previous researchers. In addition, discrimination was refined by plotting source-related and petrogenically-significant rations of elements after a rigorous element mobility analysis. Th/Zr, V/Ti. U/Nb, and V/Y proved to be particularly useful for discriminating between the two bentonite begs. Analysis of these ratios and others not only provides geochemical signatures, but can be used to describe general volcanic processes and possible magmatic source composition changes overtime. This study indicates that geochemical discrimination of Two medicine bentonites can be used to generate a high-resolution stratigraphic framework, and this in turn can be employed to frame a variety of paleobiological questions.

Magnetostratigraphy of the Upper Cretaceous Maevarano and Berivotra Formations, Mahajanga Basin, Northwestern Madagascar

The Maevarano Formation and overlying Berivotra Formation cap the Upper Cretaceous section in the central Mahajanga Basin in northwestern Madagascar. The Berivotra Formation, which is marine in origin, yields an assemblage of fossils consistent with a Maastrichtian age (~65Ma). The age of the Maevarano Formation is presently based on correlation with the Berivotra Formation. In an effort to test and further refine the current age interpretations I analyzed the paleomagnetic signal from a suite of samples collected from both formations using alternating field demagnetization. The results show a long interval of normal polarity followed by a single reversal located in the uppermost part of the Berivotra Formation. This simple pattern could place this stratigraphic section any number of places within the global polarity time scale, but fortunately the Cretaceous-Tertiary Boundary provides an independent datum in the local section. Preliminary results of this magnetostratigraphic analysis suggest that the Berivotra Formation and the underlying Maevarano Formation are late Maastrichtian in age. Additional samples throughout the Maevarano and Berivotra Formations as well as a conglomerate test are needed to test the validity of this interpretation.

Calcite Twin and Anisotropy of Magnetic Susceptibility Analyses of Ultramafic Lamprophyre Dikes: A Study of Strain Orientations and Flow Fabrics

Oriented samples were taken from two perpendicularly oriented ultramafic lamprophyre dikes in Little Presque Isle near Marquette, Michigan. The dikes intrude Archean gneisses, which are unconformably overlain by the Jacobsville Sandstone, and are presumed to be Keweenawan age. Magmatic calcite grains and groundmass are preset in a mineral assemblage of pholgopite and Fe-rich oxides. The two dikes strike east-west and north-south, with the north-south dike being the younger of the two as established by crosscutting relations. Eleven oriented samples (4 E-W, 7 N-S) were collected and 29 thin sections were analyzed using the calcite strain-gauge technique. Analysis of the E-W dike (2 samples, 4 thin sections, 89 grains, 26.97% NEVs) reveals a sub-horizontal dike-acute shortening strain fabric (N54ºE, 1º). Analysis of the N-S dike (7 samples, 7 thin sections, 10 grains, 22.43% NEVs) reveals a sub-horizontal and dike-parallel strain fabric (N7ºE, 6º). The NEVs from both dikes represent a possible secondary sub-vertical shortening strain event. In addition, analyses of dike-parallel calcite veins from both dikes reveal as NE-SW sub-horizontal shortening

Stress-Strain Analysis in Precambrian Quartzites from Wisconsin: Evidence for Eastward Continuation of the ca. 1650 Ma Mazatzal and Central Plains Orogenies

"Baraboo interval" quartzites (BIQ) deposited throughout Wisconsin between 1750-1630 Ma were analzyed using three independent strain gauge techniques, including the auto-correlation function (ACF) within current NIH Image software. The ACF has been used to quantify fabric intensities, and is here used as a proxy for bulk strain. Strain ellipse orientations (which parallel regional fold orientations) suggest approximately north-directed shortening along a roughly east-northeast trending orogenic margin. The strain gradient preserved in these rocks decreases to the north, consistent with a post-Penokean thrust margin located south of present-day Wisconsin. These analyses provide evidence of Early-Mid Proterozoic deformation in Wisconsin, contemporaneous with the Mazatzal-Central Plains (southwest) and Labrador (northeast) orogenies that partially define the southern margin of the North American craton at 1650 Ma. Until recently, when more accurate depositional dates were obtained for the BIQ, deformation in the region was usually attributed to the Penokean Orogeny (ca. 1850 Ma) along the east-west trending Niagara fault north of the BIQ. This research shows that although strain orientations are complementary with the Niagara fault, such a relationship would not be possible with the observed strain gradients.

Comparative Sedimentology of Two Late Cretaceous Localities near New Ulm, Minnesota

Upper Cretaceous strata are preserved between Precambrian bedrock and Pleistocene glacial deposits in the Minnesota Valley Minerals (MVM) Courtland and Ridgley clay and gravel quarries near New Ulm, Minnesota. These exposures provide rare glimpses into the enigmatic eastern margin of the Western Interior Seaway. Field observations of outcrops were coupled with laboratory analyses (including X-ray Diffractometry, shale disaggregation, sandstone sieve analysis, and petrography) and were used to characterize the nature of the depositional environment for each locality.

Three distinct facies were identified in the MVM Courtland mine. The Lower Laminated Claystone (LLC) facies contains virtually no coarse-grained (>coarse silt) particles. Fine (mm- to cm- scale) color bands, scattered well-preserved leaves, and siderite concretions also characterize the LLC, which is interpreted to reflect deposition in a large, open intracratonic lake. A fine-grained, cross-bedded quartz arenite body comprises the Sandstone facies (SS) and caps the LLC in three of four measured sections. The SS facies includes a basal lag deposit with imprints of carbonaceous material and is interpreted to be a channel deposit. The Upper Massive Claystone facies (UMC) crops out above the SS facies in one of the four sections. Based on apparent soft-sediment deformation structures on the contact between the SS and UMC facies, the UMC is interpreted to be part of the same channel deposit as the SS facies. The juxtaposition of a high energy channel environment on an open lake environment is unconformable, and thus might reflect a channel avulsion event or a fall in sea level.

The MVM Ridgley mine is dominated by inclined heterolithic stratification (IHS) deposits characterized by gently dipping 5-10 cm-thick packages of alternating sandstone and claystone. Sandstone units are fine-grained and shows tabular cross-bed sets with tangential toes and carbonaceous drapes on foresets. Claystone units are massive. A dark, lignitic mudstone layer caps Cretaceous exposure at Ridgley. Ridgley deposits as a whole are likely the result of deposition in a tidally influenced meandering river system.

Detailed sedimentologic anlysis of limted Cretaceous nonmarine and marginal marine exposures in southern Minnesota afford a better understanding of the eastern margin of the Cretaceous Western Interior Seaway.

Character and Significance of a Silicified Unconformity in Late Triassic-Early Jurassic Strata of the Limpopo Valley, Southern Zimbabwe

The Limpopo Valley of southern Zimbabwe preserves well-exposed strata spanning the Permian to Early Jurassic, including the Fulton's Drift, Gushu, Mpandi and Samkoto Formations, capping the sedimentary succession are the Tuli Basalts. Despite years of intensive study of the South African Karoo system, the Limpopo Valley has remained largely unexplored from both a paleontological and geological perspective.

Our recent fieldwork in this area resulted in the discovery of a previously unrecognized, laterally continuous erosional unconformity embedded between the Mpandi Formation and superjacent Samkoto Formation. This unconformity, which is characterized by scour topography, rests upon the 2-m-thick silicified cap of the Mpandi Formation. XRF analyses reveal an increased concentration of SiO2 in the silicified layer, and depletion of major and trace elements in the silicified layer compared to the background Mpandi sediments. XRD analyses of clay minerals indicate an absence of a deep kaolinite or illite weathering profile directly below the silcrete. The lateral continuity and characteristic profile of the layer (globular base grading up into prismatic and pseudobreccia horizons) suggest formation at the surface as a result of pedogenic processes. Within a predominantly GS and M-Fabric matrix, length fast and length slow chalcedony are present, with vug fills dominated by microcrystalline quartz, length fast chalcedony, and macrocrystalline quartz. Illuviation structures such as colloform features typically formed by the movement of groundwater are noticeably absent.

Geochemical data, petrographic evidence, and field relationships indicate pedogenic formative processes active in an arid to semi-arid climate regime. This interpretation is consistent with studies of the South African Karoo system that indicate a Late Triassic-Early Jurassic regional shift to an increasingly arid climate regime under stable tectonic conditions. The unconformtiy, along with a new radioisotopic age date from the Tuli Basalts, provide much neeeded age control for the Limpopo Valley succession, especially as it pertains to correlation with South African Karoo strata.

A Fusion Based Method of Whole Rock Dissolution for ICP-Mass Spectrometry and the Origin of Midcontinent Rift Granophyres

Inductively coupled plasma-mass spectrometry (ICP-MS) is a preferred method of rare earth element analysis but requires complete sample dissolution to obtain accurate results. Low pressure, low temperature acid digestion is a common method of dissolution employed in sample preparation, but refractory minerals (e.g. zircon) may not completely dissolve. Experiments performed at Macalester College have shown fusion with a lithium metaborate flux and subsequent dissolution in dilute nitric acid to be a reliable and efficient method of sample preparation for chemically resistant rocks such as granites. For the granitic rocks of the Midcontinent rift, experimentation showed a rock:flux ration of 1:4 (0.4 g rock powder) to consistenly result in complete dissolution of the sample while minimizing sample dilution and sampling error. Trace element analyses of several international standards (AC-E, STM-1, DTS-1, BIR-1, G-2, W-2) using this method of sample preparation are in close agreement with published values (<5% relative for many elements including the REE) and indicate the accuracy of this method.

This method was then applied to the granitic complexes of the Midcontinent Rift (MCR), commonly termed granophyres. The granophyres consist of basal diorite and monzodiorite and progress upward to quartz monzodiorite, granodiorite, and granite (Kennedy, 200). This study focused on the petrogenesis of four of these comnplexes: the Greenwood Lake, Misquah Hills, Eagle Mountain, and Pine Mountain granophyres.

The four granophyre complexes addressed in this study have U-Pb zircon ages that fall into two distinct groups. The older granophyres include the Misquah Hills and Greenwood Lake complexes and have been termed "early stage granophyres". The younger granites include the Eagle Mountain aned Pine Mountain bodies and have been termed "main stage granophyres". The granophyres are grouped according to the stage identified by Miller and Vervoort (1996). Isotopic and trace element data suggest both fractional crystallization (FC) and assimilation fractional crystallization (AFC) processes were involved in the evolution of the granophyres. The main stage granophyres have low Nb/Y ratios, a signature of contamination by crustal materials, whereas the Nb/Y ratios of the early stage granophyres are positively correlated with La/Sm. The main stage granophyres are also isotopically enriched (Epsilon Nd -3 to -8) suggesting assimilation whereas the early stage granophyres are not enriched (Epsilon Nd 0- to -2) suggesting that if assimilation did occur, older, enriched crustal materials were not assimilated. These data support the model of rift evolution as presented by Vervoort and Green (1997).

Paleosols as Indicators of Paleoclimate in the Upper Cretaceous Maevarano Formation, Mahajanga Basin, Northwestern Madagascar.

The ~80m thick Masorobe Member of the Maevarano Formation preserves abundant paleosol profiles that are characteristically red (typically 10R 5/6) with drab gray to gray-green mottling that tracks sub-centimeter-scale root casts surrounded by 1-2-cm-thick reduction halos (typically 5GY 8/1). Root mottling in the Masorobe paleosols is predominantly vertical in orientation, with some root casts extending up to 50 cm down-profile. Pedogenic carbonate in the form of coalesced nodules and/or carbonate-encrusted rhizoliths occur in 36% of the sampled paleosols (13 out of 36 units display macroscopic carbonate accumulations), and is most abundant in the upper two-thirds of the Masorobe section. Paleosols of the Masorobe Member lack clearly defined peds or grain-coating argillans, and the minimal development of these features suggest that these paleosols are relatively immature. XRD analyses confirm that smectitic clays dominate throughout the Masorobe Member. Geochemical analyses (XRF) of two selected paleosol profiles reveal zones of accumulation of mobile elements such as Na and K (B horizons), as well as accumulations of Al (coupled with relative decreases in Si) that are interpreted to reflect horizons of clay accumulation (Bt horizons). One 2.25-m thick composite paleosol shows three discrete zones of accumulation.

Paleosols of the Masorobe Member can be classified as either Protosols or Calcisols, depending on carbonate content. Data indicate that these paleosols developed in a well-drained, semi-arid setting where water was present in sufficient quantity to mobilize soluble elements but not entirely leach them from the system. The presence of nodular carbonate horizons >15 cm thick suggest that the mean annual precipitation on the Masorobe alluvial plain was probably less than 760 mm. This paleoclimatic reconstruction is consistent with other aspects of the sedimentology and taphonomy of the Maevarano Formation.