The origin of clays and clay minerals in the Paleocene Clayton (CF) and Porters Creek (PCF) Formations within the Mississippi embayment of central North America has been debated for more than 50 years. X-ray diffraction and petrographic analysis of samples of the CF and PCF from a mine in southeastern Missouri are used to evaluate contributions from Cretaceous-Paleogene (K-Pg) impact debris and the role of sediment diagenesis in the fine-grained sediment. Expandable clay minerals increase in abundance relative to illite and kaolinite above the K-Pg unconformity in the CF and PCF, and include dioctahedral smectite, vermiculite, and minor mixed-layered clay components, along with trioctahedral smectitic clays in the CF and lowermost PCF. Additional diagenetic phases include clinoptilolite (in the CF and lower PCF), pyrite, siderite, and opal CT (mainly in the PCF). The results of the petrographic analysis show no evidence for volcanic ash contributing directly to the sediment in the PCF. The detrital silicate minerals are mainly quartz, muscovite, biotite, and metamorphic minerals, consistent with an ancestral Appalachian Mountains source rather than volcanic ash or a Cretaceous western interior sediment source. The illite, kaolinite, dioctahedral smectite, and ordered illite-smectite mixed-layered clays are present in varying quantities in Cretaceous through Paleogene marine and nonmarine mudstones from the Mississippi embayment, and appear to be detrital in origin. Trioctahedral smectite and clinopti-lolite in the CF and lowermost PCF are argued to derive from alteration of glassy impact debris; clasts in the basal CF contain microtektites replaced by trioctahedral smectitic mixed-layered clay with randomly interstratified illite. The X-ray diffraction characteristics of the vermiculite in the PCF indicate a hydroxyl-interlayered aluminous variety that is argued to have a diagenetic origin, formed by clay mineral reactions under variably anoxic conditions in the PCF sediments during early diagenesis. Throughout the CF and PCF, pyrite precipitated under reducing conditions during diagenesis, locally along with siderite replacement of micritic carbonate. In addition, opal-CT precipitated as a result of silicate reactions and dissolution of diatoms under alkaline conditions and replaced matrix and calcitic microfossils throughout much of the upper PCF.

Eight climatic events can be distinguished in the Triassic–Jurassic (ca. 201 Ma) continental strata of Poland. These events are distinguished by kaolinite/illite ratio, chemical index of alteration (CIA), color of sediments, and palynomorphs. The first transition to wetter climate, evidenced by a shift from smectite- to kaolinite-dominated mudrocks, coincides with the earlier (“precursor”) Rhaetian negative δ13Corg excursion, which means that the beginning of climate perturbations predates the oldest known Central Atlantic magmatic province flood basalts by some 100–200 k.y. The later global, late Rhaetian “initial” negative δ13Corg excursion is divided into two subpeaks, each corresponding to hot and humid events, separated by a cooler and drier event. The upper subpeak is also associated with perturbation of the osmium isotope system (attributed to volcanic fallout), and darkened miospores, pointing to acid rains. Between the “initial” excursion and the Triassic-Jurassic boundary interval, five climatic fluctuations are inferred from the changing kaolinite/illite ratio, the last two of which are also associated with an Os isotope perturbation, polycyclic aromatic hydrocarbon (PAH) occurrences, a “spore peak,” and darkened miospores. A series of periodic atmospheric loading events by CO2, CH4, or alternatively by SO2, sulfate aerosols, and toxic compounds, is inferred to have caused this series of rapid climatic reversals and resulting extinction of many less-adapted forms. Just above the palynofloral extinction level, appearance of new forms commenced Jurassic palynofloral recovery. Tetrapod evolution events in the end-Triassic–earliest Jurassic were related to the extinction of the Pseudosuchia, Dicynodontia, Capitosauroidea, Plagiosaroidea, and Rhynchosauria, while appearance of highly diversified tetrapod ichnofauna in the earliest Jurassic strata indicates a rapid recovery and refill of ecological niches by dinosaurs.

This field guide describes four exposures of glacigenic sediment along the Missouri River bluffs east of Omaha, Nebraska. Field trip stops include Loveland, Iowa, which is the type section of the Loveland Silt and Pisgah Formation (Illinoian and Early Wisconsinan loess) and Crescent Quarry, which exposes the type Nebraskan till. Additionally we will examine core samples of the Kennard Formation, a new stratigraphic unit consisting of multiple pre-Illinoian loesses. We also present recent results on the pre-Illinoian till stratigraphy in the Missouri River valley region. A variety of evidence indicates that the present location of the Missouri River valley originated sometime between deposition of the youngest (pre-Illinoian) till and the (Illinoian) Loveland Silt. The spatial distribution of the youngest pre-Illinoian till further suggests that this reach of the Missouri River (along the Iowa-Nebraska border) was established as an ice-marginal stream along the western terminus of the last pre-Illinoian glaciation.

This work represents an integrated analysis of weathering landforms, including minor landform morphologies and soil profiles developed on granitoid terrains of the Sila Massif uplands (Calabria, southern Italy). The results of our analysis indicate that cryoclastic and thermoclastic processes, along with chemical weathering, are the main factors controlling rock degradation. Microscale features observed in primary minerals and parent rock fabrics, such as structural discontinuities, cleavage planes, fracturing patterns, and variations in chemical composition, play important roles in triggering weathering and, given sufficient time, progressively lead to grussification and soil development. Exfoliation, hydration, and splitting apart of biotite, as well as hydrolysis and etching of plagioclase and K-feldspar, appear to be prominent factors in the breakdown of bedrock. Whereas time controls the degree of development of the main weathering features and climate infiuences type and intensity of the dominant processes, relief strongly influences the development and preservation/removal of the regolith/soil cover. Geomorphological evidence of severe surface erosion is quite good, especially along steep slopes where weathering products are quickly removed, although on the highest, dissected paleosurfaces (the oldest paleolandscape remnants in the Sila Massif), wide boulder fields represent relics of past, deep spheroidal weathering that have been exhumed by intense erosion. Erosive, depositional, or reworking phenomena, often enhanced by human activity, are well recorded by macro- and micromorphological features of soils, which show simple, poorly differentiated, rejuvenated profiles, buried or truncated horizons, abundant coarse-grained primary minerals or rock fragments, and pedorelicts. The soil clay mineralogy, characterized by illite, chlorite, and vermiculite, and the dominance of coarse textures confirm a young pedogenetic stage of evolution, although highly weathered sand grains (quartz included) occur in rarely preserved mature paleosols. This interpretation is also consistent with the compositional immaturity of fiuvial sands, which have undergone low to moderate transport.

A vertical section of Upper Paleozoic sandstones from southern Transdanubia (Mecsek-Villány area, Tisza mega-unit, Hungary) has been analyzed for major and trace elements, including rare earth elements (REEs). In addition, the clay mineralogy of the sandstone samples and the petrography and geochemistry of gneiss and granitoid clasts extracted from the associated conglomerates have been determined. Geochemistry of the sandstone samples analyzed in this study shows that these rocks were predominantly derived from a felsic continental source; nevertheless, compositions vary systematically up-section. The Pennsylvanian (Upper Carboniferous) Téseny Formation has higher SiO2 and lower Na2O, CaO, Sr, high field strength element (HFSE), and ΣREE contents relative to the Permian strata. Its high K2O and Rb contents together with the presence of abundant illite-sericite suggest a potassium metasomatism in this formation. Clay mineralogy and large ion lithophile element (LILE) contents of the Lower Permian Korpád Formation vary spatially and are interpreted as local variations in composition of the source region and postdepositional conditions. Zr and Hf abundances and REE patterns, however, show that this formation was derived from mature upper continental crust. The Upper Permian Cserdi Formation has higher TiO2, Th, U, Y, Cr, and heavy (H) REE contents, and higher Cr/Th and Cr/Zr ratios relative to the underlying formations. These trends can be explained by a sedimentary system dominated by highly weathered detritus derived from combined recycled-orogen, basement-uplift, and volcanic-arc provenance in the Téseny Formation, with an increased proportion of less weathered detritus derived from combined volcanic and basement-uplift provenances in the Permian formations. Characteristics of the Cserdi unit may reflect relatively proximal derivation from a felsic volcanic source.

Monte Alpi represents the only area of the southern Apennines where Apulian carbonates, elsewhere buried beneath a several-km thick allochthon, are exposed at the surface. These rocks also represent the reservoir interval in southern Italy's major oil fields. The tectonic evolution of this substantially exhumed area of the fold-and-thrust belt, derived from conventional structural and stratigraphic considerations via integration into the regional framework, has been tested and detailed by the analysis of vitrinite reflectance, clay mineralogy, apatite fission track, and fluid inclusion data. The Apulian carbonates of Monte Alpi underwent significant tectonic burial as a result of thin-skinned thrusting in early Pliocene times. Simplified burial and thermal modeling suggests that the thickness of allochthonous material emplaced on top of Monte Alpi was probably in excess of 5 km. Exhumation is envisaged to have started in the late Pliocene, when the area emerged and the tectonic load started to be eroded off Monte Alpi. A significant stage of exhumation is inferred to have taken place in uppermost Pliocene-early Pleistocene times as a result of thick-skinned reverse faulting at depth and coeval thin-skinned extension within the overlying allochthon. After shortening ceased throughout the whole southern Apennines, middle Pleistocene-Holocene tectonic exhumation of Monte Alpi was essentially controlled by thick-skinned extensional tectonics. This process is still active and controls the present-day seismicity of the study area.

In order to investigate if high elevations existed in the Rocky Mountains before the late Miocene, we examined oxygen isotope ratios of 63 Tertiary smectite samples as a proxy for the isotopic composition of precipitation. Of these samples, 51 were also analyzed for hydrogen isotope ratios. These smectites were formed as a result of the weathering of volcanic air-fall deposits that blanketed much of western North America during the Tertiary. Smectite-bearing ashfall samples were collected from Eocene, Oligocene, and Miocene deposits along a transect that extends from the western Great Plains to Yellowstone National Park at modern elevations from ∼900 to ∼2800 m. In general, oxygen and hydrogen δ values of smectite lie along a line parallel to the meteoric water line, which suggests that the isotopic composition of these ash-derived smectites records the meteoric water composition during its formation. There is little evidence for postdepositional exchange with basinal brine fluids, evaporative effects, or diagenesis of these smectites. The δ18O values of Oligocene and Miocene samples increase ∼6‰ linearly from sample sites located at the crest of the Rocky Mountains to sites in western Nebraska and South Dakota. These results mimic the distribution and values of calculated oxygen isotope ratios of theoretical modern smectite over this same geographic traverse of decreasing elevation. This result suggests modern atmospheric circulation patterns and that the resulting distribution of δ18Oprecipitation has persisted since the Oligocene. The δ18O values of Eocene samples increase ∼8‰ between the Yellowstone region and central Wyoming, a result that does not correlate with modern δ18Oprecipitation trends. Our Eocene results may be explained by climate conditions extant at that time, but tectonic modification in the region between 50 Ma and 37 Ma cannot be excluded as the cause of our results. Because the modern climate system requires interaction with and modification by high-elevation areas, our results suggest that the Rocky Mountains have been at high elevation since at least 50 Ma.

In many young orogens of the Mediterranean region, Quaternary tectonics and regional uplift are traditionally considered strictly correlated, but few data are actually available for precise calculations of uplift rates. Such quantitative studies are needed to define the relationships between local faulting and large-scale uplift, and to formulate correct hypotheses on the geodynamic scenario in which the regional raising occurred. In addition, data about burial depths of sediments or tectonic loadings suffered by sedimentary and low-grade metamorphic rocks are essential for comparisons with the uplift rates obtained in the same areas. Such comparisons between quite different data sources improve the comprehension and choice of the most reliable mechanism responsible for the regional uplift and exhumation. Uplift rates have been calculated for a large sector of the Lucanian Apennine (axial zone of the southern Italian Apennines), and also reviewed for the Calabrian arc (southern termination of the Italian peninsula), using both geomorphological observations (elevation values, ages, and arrangement of depositional and erosional land surfaces and other morphotectonic indicators) and stratigraphical and structural data (sea-level related facies, base levels, fault kinematics, and offset estimations). Such data have been compared with those derived from clay mineralogy of Mesozoic pelagic deposits (Lagonegro units), outcropping in the same sector of the chain, which give information on burial depths. The values of the Quaternary uplift rates of the southern Apennines axial zone vary from a minimum of 0.2 mm/yr near the town of Potenza to a maximum of ∼1.2–1.3 mm/yr in Agri high valley, a severely deformed Quaternary intermontane basin, still tectonically active, in the Pollino Mountains, a carbonate ridge with elevation up to 2200 m above sea level (asl); intermediate values (0.5–0.7 mm/yr) have been calculated for the other studied areas. The erosion rate from a key area of the Lucanian Apennine, obtained from both quantitative geomorphic analysis and missing volumes calculations on a catchment basin as wide as 150 km2, has been estimated at 0.2 mm/yr for the middle Pleistocene to Holocene time span. Since in the upper part of that chronological interval erosion and uplift rates match well, the axial-zone landscape could have reached a flux steady-state during the late Pleistocene. Using geomorphological features and late Pliocene to Pleistocene deposits involved in the genesis of erosional and depositional land surfaces, similar rates (≈0.6 mm/yr) have been obtained for a quite large time span (∼2 m.y.) in the Melandro basin and adjacent Maddalena Mountains. Therefore, during the last 2 m.y., the total uplift amount of the axial zone of the Lucanian Apennine is ∼1.2–1.3 km, with local peaks of 1.5 km. On the other hand, the Mesozoic pelagic units experienced tectonic loading of 4–5 km, as estimated by means of illite crystallinity (in the range 0.6–1.1 ▵°2[thetas]), percentage of illitic layers in illite/smectite mixed layers (60%–90%) and white mica polytypes (in the range of 15%–35%), and confirmed by other independent data. The Quaternary uplift and the related erosion rates of the southern Apennines are unquestionably due to extensional tectonics coupled with thermal/isostatic regional raising and, in minor extent, to strike-slip faulting acting in the earlier deformational stage of the south Apennines chain. The gap of several kilometers deriving from the comparison between uplift rates and tectonic loading values may be explained only with different exhumation modalities starting from late Miocene times. This age can be obtained assuming a fixed rate of 0.6 mm/yr, which represents the best long-term estimate for the axial zone of the chain. Going back in time using such a conservative rate to get a denudation value of ∼4–5 km, Tortonian age is reached. At that time, contractional tectonics were still active in this sector of the southern Apennines. Tectonic denudation may be a reliable explanation for the discrepancy between the different sources of data. Such phenomena led to the exhumation of the Mesozoic core of the chain (Lagonegro units), causing low-angle extension on its Tyrrhenian side by reactivation and inversion of older thrusts, and stacking on its eastern margin of Cretaceous to Miocene pelagic and flysch units (i.e., frontal imbricate fan units) by gravity megasliding.

Widespread volcanic components in the eolian Page Sandstone and coeval and overlying Carmel Formation (fluvial, eolian, sabkha, restricted marine) in south-central Utah and adjacent Arizona document extensive Middle Jurassic volcanic activity in the adjacent Cordilleran arc. The most unusual of these volcanic components consist of pure to sandy bentonite fill within trough-shaped eolian scours or super scoops encased in cross-stratified eolian dune deposits. More than 200 of these deposits have been identified in the Page Sandstone near Page, Arizona. Also present are several regional but irregular and discontinuous, thin (several cm) bentonite beds associated with sandy sabkha and eolian sandsheet deposits. Volcanic components are also present as detrital sand- to boulder-size grains in both the Page Sandstone and Carmel Formation. Most of the identified clasts consist of rhyolite tuff fragments. Sand-size clasts occur in fluvial and eolian deposits and cobbles to boulders occur in bedload and mudflow deposits associated with ephemeral stream channels. Bentonites accumulated during brief to possibly extensive lulls in sedimentation. Those with pure composition suggest little to no reworking and rapid accumulation, probably in one ash fall. Volcanic clasts were eroded and transported from extensive rhyolite-tuff aprons in and adjacent to the arc, northward, onto alluvial and coastal plains of the study area. A sharp, northward decrease in grain-size of the largest boulders documents that some volcanic aprons extended to within several kilometers of present Jurassic outcrops. X-ray diffraction (XRD) analyses reveal that the bentonites are predominantly authigenic, long-range ordered, mixed-layered illite/smectite (80%–90% illite, Reischeweite 1–3; Reischeweite is explained in the “Clay Mineralogy” section). Bentonites in underlying and overlying formations occur as randomly interstratified illite/smectites with no more than 60% I. Mineralogy of Page bentonites is not produced by regional burial diagenesis, but is a product of local, near-surface diagenetic environment. These bentonites document geochemical alteration of volcanic ash by highly saline, potassic waters in a cyclic wet/dry hydrologic regime. Regional stratigraphic analysis of the Page Sandstone and the coeval and overlying Carmel Formation document the following tectonic events in the region: (1) lower and middle Page—long-term, sporadic accumulation of volcanic ash on a stable, arid coastal plain with increasing fluvial activity late in this episode; (2) upper Page—increased volcanic activity and local rapid subsidence (development of foredeep adjacent to arc uplift); and (3) uppermost Page, overlying Carmel—strong influx of volcanic detritus and reworked sedimentary material. These events document the well-known Middle Jurassic ignimbrite flare-up (ca. 170 Ma) and strong, possibly localized uplift near the southern margin of the Colorado Plateau.

Channel and column samples of the Herrin and Springfield coals, as well as roof shales, were collected from four underground coal mines in southern Illinois. Coal lithotypes (vitrain, bright banded coal, subbright banded coal, dull coal, and fusain) and mineral partings were hand-picked from the channel and column samples. Chemical, petrographic, and mineralogical studies of the samples indicate that the vitrain separates have the highest chlorine (CI; mean 0.52%) and lowest sodium (Na; mean 0.088%) and ash (mean 2.8% high-temperature ash) contents. Roof shale and mineral partings have the lowest CI content (mean 0.25%) and highest Na (mean 0.263%) and ash (mean 40.6% high-temperature ash) contents. The Cl/Na ratio of the lithotypes is significantly greater than that of NaCl, the dominant dissolved salt in the ground water associated with Illinois coals. Chlorine and Na in the coal samples studied are concentrated in vitrinite and mineral matter, respectively. Part of the Na and a small fraction of the Cl occur as dissolved NaCl in macropores and mesopores, mainly within the mineral matter and inertinite. The remaining Na is adsorbed on clay minerals. The zeta potential data obtained for the same lithotypes support the interpretation that the organically associated Cl in coal is adsorbed primarily on the micropore walls. One set of lithotypes was leached with water at 27 °C and 93 °C, and with a 4.3 M NH4OH solution at 27 °C. The aqueous leachability of Cl increases from vitrain, through bright banded, subbright banded, and dull coal, to fusain. This is in the order of decreasing vitrinite content and, consequently, of decreasing micropore abundance in which most of the Cl is held. Thus, variable Cl leachabilities among the lithotypes may be related to their pore-size distributions.

X-ray diffraction analyses of till samples collected from multi-till exposures along the Lake Michigan shoreline in Michigan and Wisconsin show a systematic variation in the clay-mineral composition of successive till sheets. A general increase in the relative amount of 10Â clay occurs within a succession of till sheets, beginning with early or middle Wisconsinan Glenn Shores till and continuing through late Wisconsinan (Woodfordian) Ganges–New Berlin till and Saugatuck–Oak Creek till. A significant decrease in the relative amount of 10Â clay, however, occurs within the post–Mackinaw Interstade (late Woodfordian) Ozaukee-Haven and Two Rivers tills. These changes in clay-mineral composition are apparently related to major ice-margin fluctuations since a significant retreat of the Lake Michigan Lobe has been recognized between deposition of each of the above till sheets. Morphostratigraphic correlation of the Powell Moraine of the Erie Lobe with moraines of the Saginaw and Lake Michigan Lobes, as well as correlation between till units of the Saginaw and Lake Michigan Lobes, indicates that the retreat recorded between deposition of Ganges–New Berlin and Saugatuck–Oak Creek tills of the Lake Michigan Lobe is correlative with the Erie Interstade.

Clay mineral analyses of underclays directly below the Upper Elkhorn Coals (eastern Kentucky) and the Lower Kittanning Coal (western Pennsylvania) demonstrate systematic lateral and vertical variations that include changes in the kaolinite-illite ratio, mica loss ratio, weathering ratio, apparent thickness of mica, and distribution of chlorite. Clay mineral analyses of associated, unweathered shales indicate that approximately 30 percent of the regional variation in Lower Kittanning underclay mineralogy is inherited from the parent material. The remaining variation is attributed to in situ pedogenesis. Petrographic analysis of thin sections from a fluvial sandstone subjacent to the Upper Elkhorn underclay suggests that position of the ground-water table controlled pedogenesis. Two distinct alteration zones separated by a diffuse, subhorizontal boundary are present in the sandstone: an upper zone characterized by kaolinization of feldspars, dissolution of chlorite and detrital dolomite, and absence of siderite; and a lower zone characterized by ferron dolomite replacement of both detrital feldspar and detrital dolomite, and authigenic pore fillings of chlorite and siderite. These systematic changes in sandstone and underclay mineralogy are consistent with a pedogenic model in which the process of podzolization was controlled by position of the ground-water table and topography. In this model, the main phase of organic material accumulation occurred above the underclay after water-table levels intersected the land surface as a result of compaction, subsidence, or marine transgression. Regional gravity and structure data in western Pennsylvania further suggest that syntectonic movements were the fundamental controls on regional topography and ground-water levels, and thus, pedogenesis.

Paleosols are well preserved beneath Upper Cretaceous continental sediments from the eastern Gulf Coastal Plain to the northern Mississippi Embayment. These buried soils are remnants of land surfaces that weathered during Cretaceous time and formed on several rock types, including Precambrian(?) metamorphic rocks and Paleozoic limestone and sandstone. Most soil profiles have distinctive morphologic features and exhibit pedogenic horizon zonation with ferruginous and clay-rich B horizons grading downward to saprolite and parent material within 7 m. Organic matter is not preserved, but pedotubules, which we interpret as fossil roots or burrows, are present in many paleosols. Illuvial and residual concentrations of Al2O3 and Fe2O3 occur in upper soil horizons where kaolinite and halloysite are the principal clay minerals. A variety of precursor minerals, including 2:1 lattice clays, muscovite, feldspar, and biotite, are present in the lower part of the soil profiles. Variations in pedogenesis were caused by differences in parent material and by local geomorphic factors such as paleoslope and drainage. The rates of paleosol development and the residence times of the various land surfaces are uncertain, but the times of burial and preservation range from Cenomanian to early Maestrichtian. Chemical and mineralogic characteristics of these paleosols are similar to those of modern soils of tropical to subtropical climates; however, the fossil soils may be polycyclic, and their pedogenic characteristics could have been modified by postburial diagenesis. Climatic reconstructions based on paleobotanical, lithostratigraphic, and paleotemperature data from Cretaceous sediments support our interpretation that these paleosols formed in a warm, humid environment on well-vegetated piedmont surfaces.

An organic rich, gleyed soil (Cumulic Haplaquoll) on the edge of a sag pond near San Marino High School illustrates the effects of poor drainage and tectonism on soil formation (pedogenesis) in a semiarid region during the Holocene. Combined with other characteristics of soil development, the organic matter masks the presence of six depositional strata in the granodioritic alluvium in which the soil formed. Two of these units occur only on the south side of a strand of the Raymond fault, three occur only on the north side, and one occurs on both sides. The dominant clay minerals are smectites that have formed from soil solution within the past 10,000 years. Beidellite formed in horizons with exchangeable aluminum (pH less than 6.5), and montmorillonite formed in horizons without exchangeable aluminum (pH greater than 6.5). A soil tongue produced by fault movement was relatively unweathered after it was emplaced by an earthquake that occurred less than 1,400 years ago. Compared to other Holocene soils, pedogenesis at this site is intermediate between the weakly developed Hanford soils formed in granitic alluvium in the San Joaquin Valley and the strongly developed Concepcion soils formed in continental terrace deposits along the coast.

The iridium (Ir) anomaly at the Cretaceous/Tertiary (C/T) boundary can be explained by dissolution of normal pelagic limestones producing an insoluble clay residue containing Ir-rich meteoritic material. Further concentration of Ir and other trace metals in the clay can be explained by deposition under reducing conditions. Clay-mineral analyses indicate that the boundary clay is similar to locally derived clays in the limestones above and below the boundary, and contains no detectable exotic components. This supports a terrestrial origin for the boundary clay. The genesis of the boundary clay is attributed to changes in ocean chemistry (pH and oxygenation), leading to an interval of widespread dissolution of carbonate sequences in the oceans, and culminating in the development of submarine pyritic hardgrounds and a disconformity of global extent.

Scanning electron microscope studies of particulate matter in Cretaceous/Tertiary boundary sediments reveal no common carrier phase larger than about 1 μm for the supposedly extraterrestrial elements. Particles possibly related to an extraterrestrial impact are Fe-rich aluminosilicate spheres (similar to microtektites), Al-rich smooth spheres, K-rich feldspar spheroids, and Fe-Ti–rich particles. Mercury abundances in bulk sediment samples show no consistent enrichment pattern. Some of the supposedly extraterrestrial elements (e.g., Pt, Ni) are present within diagenetic sulfide grains. Terrestrial processes have modified the extraterrestrial element signature, and characterization of an impactor simply from sediment chemistry is erroneous. Deep-sea sediments not enriched in chalcophiles will probably provide the best clues for impactor identification.