Sacramento Mountains Research Articles

Abstract: Depositional, Diagenetic, and Sequence Stratigraphic Facies Analysis of Phylloid Algal Mound Complexes within the Upper Pennsylvanian, Lower Holder Formation, Sacramento Mountains, New Mexico 

Abstract: Depositional, Diagenetic, and Sequence Stratigraphic Facies Analysis of Phylloid Algal Mound Complexes within the Upper Pennsylvanian, Lower Holder Formation, Sacramento Mountains, New Mexico 

Tectonic control on sea-floor relief and the localization of Lower Mississippian Waulsortian mounds, New Mexico

Lower Mississippian carbonate mud-rich bioherms, generally referred to as Waulsortian mounds, are commonly associated with low-paleolatitude carbonate ramp settings and have recently been recognized as important hydrocarbon reservoirs. The factors controlling localization of Waulsortian mounds have heretofore been poorly understood. Stratal relations exposed in the Alamogordo Member of the Lake Valley Formation in the Sacramento Mountains of New Mexico illustrate the effects of tectonism on carbonate sedimentation prior to, during, and after mound growth. They indicate that mound initiation and localization were strongly controlled by tectonically generated, intraramp, sea-floor topography. These observations bear strongly on understanding the controls on localization and growth of mud mounds in general. Stratal geometries observed in the underlying Andrecito Member indicate that this topography was modified by erosional and depositional processes prior to mound initiation. Mounds formed on the surfaces and margins of the intraramp topography as the result of aggradational, in situ accumulation of biogenic sediment. Differences in growth geometry of stratal units within individual mounds and differences between mounds are correlated with position of the mound on the ramp and the deformation occurring immediately prior to mound growth. It is probable that local tectonism continued during mound growth, and that local differences in the amount of relative uplift resulted in different amounts of space for growth of individual mounds, and thus determined differences in mound size and geometry.

Exceptional Preservation of the Sponge Fissispongia tortacloaca from the Pennsylvanian Holder Formation, New Mexico

Exceptionally preserved aragonitic demosponge material occurs within the Virgilian Holder Formation, Sacramento Mountains, south-central, New Mexico. Fissispongia tortacloaca (King) is identified, a species previously known only from completely calcified specimens. Standard optical microscopy and SEM examination of broken surfaces of the F. tortacloaca skeleton distinguishes between primary spherulites and a later epitaxial overgrowth. F. tortacloaca biomineralization is readily comparable to that of the modern agelasid demosponge Astrosclera willeyana, thus confirming the demosponge affinity of this form. Cathodoluminescence microscopy and back scatter SEM examination identifies selective silicification and calcitization of the spherulite centers. These diagenetic minerals affect the initial spherulite biomineralization mimicking the original microstructural fabric. Elemental composition ofF. tortacloaca (Sr = 7600 ppm, S = 1640 ppm and Mg = 150 ppm) is close to that of modern A. willeyana (Sr = 10,000 ppm, S = 1780 ppm and Mg = 320 ppm). Sr concentration in F. tortacloaca indicates that late Pennsylvanian seawater had slightly less Sr (7 ppm) than modern seawater. S is interpreted as having been incorporated into sponge aragonite as SO42-. The stable isotope composition of F. tortacloaca (8 13C = +5.5%; 8 180 = -2. 1%o) is compatible with other Virgilian skeletal carbonates from the USA, but is about 1%o heavier than A. willeyana (813C = +4.4%o; 8 180 = -0.8%o). The 8 180 composition of F. tortacloaca indicates tropical to subtropical temperatures between 24-30° C using Grossman and Ku's expression T° C = 20.6 4.34 (8 180A d: assuming the Pennsylvanian ocean had a 8 180 composition similar to modern seawater with an 1.2%o variation due to glacial ice volume effects.

Growth history of Lower Mississippian Waulsortian mounds: Distribution, stratal patterns, and geometries, New Mexico

The factors controlling the localization and growth of Lower Mississippian Waulsortian mounds have been difficult to establish because of limited exposure of individual mounds and mound-bearing platforms in western Europe, where the Waulsortian facies have been studied most intensively. Mounds on the Lower Mississippian homoclinal ramp of the Lake Valley Formation in the Sacramento Mountains, however, are exposed exceptionally well at platform, outcrop, and mound scales in an area roughly 5 km by 20 km, and provide the opportunity to better understand these aspects of Waulsortian mounds.

Hydrogeothermal studies across the Pecos River Valley, southeastern New Mexico

Heat-flow values calculated along a profile in southeast New Mexico are used to study the regional ground-water flow pattern. The west-east profile extends from the Sacramento Mountains, across the southern part of the Roswell artesian basin, to the New Mexico–Texas border. The heat-flow data are generally calculated from bottom-hole temperatures estimated at various depths, and are therefore thought to reflect depth-dependent anomalies (principally ground-water flow). Two major positive heat-flow anomalies, interpreted to indicate upward ground-water flow, are present along the profile. A narrow anomaly in the western part of the profile is consistent with upward ground-water movement along the western boundary of a buried Precambrian horst. A rather broad anomaly in the middle of the profile appears to be associated with cross-formational flow and discharge to the Pecos River. Both anomalies may be associated with upward flow from depths of 915 or 1220 m and possibly 1524 m in the western region, but probably not deeper than 1829 m. Vertical ground-water flow appears to be absent just east of the Mescalero Ridge; farther east on the High Plains, the data are generally consistent with downward ground-water flow at depths of 610–1220 or 1524 m.

Episodic growth of a Waulsortian buildup: the Lower Carboniferous Muleshoe Mound, Sacramento Mountains, New Mexico, USA

Abstract Muleshoe Mound is a composite Waulsortian buildup that crops out along the western escarpment of the Sacramento Mountains, New Mexico. Exceptional exposures of Muleshoe Mound have enabled a detailed stratigraphic model to be developed, subdividing the buildup into five separate stratal units. Each stratal unit differs in facies, geometry and symmetry, reflecting changes in the character and intensity of different environmental conditions (energy, carbonate production, oxygenation and accommodation space). Hiatal surfaces mark periods of mound crisis and/or sea-level fluctuations. These hiatal surfaces highlight the episodic nature of the mound’s growth. Episodic growth acted as an important control on this buildup’s development, facies and early diagenesis. Features and patterns of the Muleshoe Mound conflict with the traditional interpretation of the Waulsortian as quiet, stable, deep-water buildups. Muleshoe Mound appears to have grown in appreciable currents where accommodation space was a crucial control on growth.

The Effect of Relative Sea Level Change on the Development of Phylloid Algal Bioherms, Laborcita Formation (Lower Permian), Sacramento Mountains, New Mexico: ABSTRACT

The Effect of Relative Sea Level Change on the Development of Phylloid Algal Bioherms, Laborcita Formation (Lower Permian), Sacramento Mountains, New Mexico: ABSTRACT

Constituent composition of Early Mississippian carbonate buildups and their level-bottom equivalents, Sacramento Mountains, New Mexico

Abstract The Alamogordo Member of the Lake Valley Formation (Lower Mississippian-Tournaisian to early Viséan) was deposited on a homoclinal ramp at depths below the fair-weather wave-base and mainly below the photic zone. Depth increased toward the present southwest, and the level-bottom biota of the Alamogordo Member is present in assemblages that parallel the depth gradient (Jeffery & Stanton this volume). Waulsortian-like mounds are abundant in the Alamogordo, Nunn and Tierra Blanca Members of the Lake Valley Formation on the distal portion of the homoclinal ramp. These mounds contain essentially the same biota as the Waulsortian buildups of Europe. Alamogordo mounds are low, tabular features composed mainly of spiculiferous micrite, while Nunn and Tierra Blanca mounds are more domical and are composed largely of fenestrate bryozoan cementstone. Lithology and biota of coeval, level-bottom beds and mounds differ significantly because of habitat differences (cavities and hard substrates in and on mounds), and because of taphonomy, an example of which is the preservation of fenestrate bryozoan sheets in radiaxial fibrous calcite cements on mounds as compared with comminuted and dispersed fenestrate hash in level-bottom muds. Lake Valley mound constituents, although essentially identical to those in the European Waulsortian, do not occur in regular growth ‘phases’ like those described by Lees & Miller (1985). The difference is interpreted to mean that the New Mexican mounds lacked enough vertical relief above the sea floor in deep water to have been affected by any environmental gradients that may have existed across the entire platform.

Constraints on extension-related plutonism from modeling of the Colorado River gravity high

The Colorado River gravity high is a positive gravity anomaly coincident in location with the Colorado River extensional corridor, a zone of extreme middle Tertiary crustal extension in the southern Basin and Range. The high extends for ≈150 km along the west side of the Colorado River and has a magnitude of 10–20 mGal above the regional level. The source of this gravity high has previously been interpreted as either mafic intrusion or dense material added to the middle crust via lateral flow during extension. Detailed modeling of the gravity high is now possible due to acquisition of new gravity data that thoroughly define the shape of the anomaly at and around its highest point. The peak of the high is 20 mGal higher than the ambient level of the anomaly, and is in the southern Sacramento Mountains, California. A small (≈50 km 2 ) middle Tertiary pluton composed of mixed and mingled diorite and granite crops out in the southern Sacramento Mountains at the location of the peak of the gravity high. Surface density measurements indicate that the Miocene diorite has the highest density in this area: a maximum density of 2.91 g/cm 3 and a mean density (including measurements of mingled diorite and granite) of 2.75 g/cm 3 . This pluton is interpreted to be the minor surface expression of a larger, deeper source for the Colorado River gravity high. Forward modeling of the Bouguer gravity data is based on surface structure and density contrasts documented by recent geologic mapping and on interpretations of the deeper levels of the crust from the seismic experiments of PACE, CALCRUST, and COCORP. Interpretation of a regional profile crossing the peak of the gravity high yielded several possibilities, from which one preferred model was chosen on the basis of geologic data. The preferred model is that of intrusion of mantle-derived melt which acquired an upward-decreasing density distribution due to mixing and mingling with magma derived from the continental crust. The magma conduit is linked through the entire crust and shows no sign of offset or “decapitation” by a detachment fault system. In three dimensions, the Colorado River gravity high can be interpreted as the geophysical signature of an ≈150-km-long, 10- to 20-km-wide zone of intrusion into the area of maximum crustal extension. The magmatic contribution to extension in this region was primarily in the form of large, subvertical, dike-like intrusions, rather than sill-like or horizontally distributed underplated material. The inflated lower-middle crust, interpreted from seismic data, may be due to either (1) lateral material flow into the zone of maximum extension, which mixed and mingled with intrusions from the mantle, or (2) ponding of some intruded material between the lower and middle crust. In either case, the ultimate cause of the gravity high is intrusion of mantle-derived melt incorporated with melt derived from continental crust, which is now exposed as mixed and mingled granite and diorite in the southern Sacramento Mountains.

Eustasy, Tectonics, and Climate: Isolation of Controls on PennsylvanIan (Missourian) Cyclothems from the Sacramento Mountains (Orogrande Basin), New Mexico: ABSTRACT

Eustasy, Tectonics, and Climate: Isolation of Controls on PennsylvanIan (Missourian) Cyclothems from the Sacramento Mountains (Orogrande Basin), New Mexico: ABSTRACT

Possible extension of the Midcontinent Rift in west Texas and eastern New Mexico

The Midcontinent Rift System forms one of the most prominent gravity features in North America. The recognized geophysical anomaly extends in an arc from southern Oklahoma to Lake Superior and then into southern Michigan. The Midcontinent Rift System was active between 1185–1010 Ma, as indicated in the Lake Superior region by age determinations on intrusive igneous rocks. We suggest that the period of formation of the Midcontinent Rift was also a time of extensive igneous activity in Texas and New Mexico. This activity is represented by intrusions beneath the Central basin platform (Texas and New Mexico), intrusions which crop out at the Pajarito Mountain in the Sacramento Mountains (New Mexico), a basaltic debris flow in the Franklin Mountains (Texas), basalt flows at Van Horn (Texas), and the Crosbyton geophysical anomaly (east of Lubbock, Texas). These bodies and other bodies located by geophysical anomalies and wells drilled into mafic Precambrian rocks may be related to the Midcontinent Rift System. Alternatively this magmatism could be related to Grenville age tectonics in Texas. The mafic igneous rocks in this area form a 530 km diameter Middle Proterozoic igneous province, which formed between 1070 and 1220 Ma. Comparison of the Midcontinent Rift System and its extensions proposed here with the Mesozoic and Cenozoic African rift systems indicates that these features are of comparable scale and complexity.

Architecture and Sequential Development of a Mississippian Carbonate Platform as Exemplified by the Sacramento Mountains, New Mexico: ABSTRACT

Architecture and Sequential Development of a Mississippian Carbonate Platform as Exemplified by the Sacramento Mountains, New Mexico: ABSTRACT

Middle Proterozoic Tectonic Activity in West Texas and Eastern New Mexico from Analysis of Gravity and Magnetic Anomalies: ABSTRACT

The Precambrian history of west Texas and eastern New Mexico is complex, consisting of four events: Early Proterozoic orogenic activity (16309-1800 Ma), formation of the western granite-rhyolite province (WGRP) (1340-1410 Ma), Grenville age tectonics (1116-1232 Ma), and middle Proterozoic extension possibly related to mid-continent rifting (1086-1109 Ma). Pre-Grenville tectonics, Grenville tectonics, and mid-continent rifting are represented in this area by the Abilene gravity minimum (AGM) and bimodal igneous rocks, which are probably younger. We have used gravity modeling and the comparison of gravity and magnetic anomalies with rock types reported from wells penetrating Precambrian basement to study the AGM and middle Proterozoic extension in this area. The AGM is an east-northeast-trending, 600 km long, gravity low, which extends from the Texas-Oklahoma border through the central basin platform (CBP) to the Delaware basin. This feature appears to predate formation of the mafic body in the CBP (1163 Ma) and is most likely related to Pre-Grenville tectonics, possibly representing a continental margin arc batholith. Evidence of middle Proterozoic extension is found in the form of igneous bodies in the CBP, the Van Horn uplift, the Franklin Mountains, and the Sacramento Mountains. Analysis of gravity and magnetic anomalies shows that paired gravity andmore » magnetic highs are related to mafic intrusions in the upper crust. Mapping of middle Proterozoic igneous rocks and the paired anomalies outlines a 530 km diameter area of distributed east-west-oriented extension. The Debaca-Swisher terrain of shallow marine and clastic sedimentary rocks is age correlative with middle Proterozoic extension. These rocks may represent the lithology of possible Proterozoic exploration targets. Proterozoic structures were reactivated during the Paleozoic, affecting both the structure and deposition in the Permian basin.« less

Well Preserved, Aragonitic Phylloid Algae (Eugonophyllum, Udoteaceae) from the Pennsylvanian Holder Formation, Sacramento Mountains, New Mexico

Remarkably well-preserved fossils of the udoteacean alga Eugonophyllvm occur in the Virgilian Holder Formation, Sacramento Mountains, south-central New Mexico in a 20-cm-thick packstone bed between two thick shale beds. Original aragonite is preserved in this bed as a felt-like mesh of needles in algal thalli, in mollusc shell fragments, in sponge walls, and in botryoidal and isopachous marine cements. The average Sr content of the algal aragonite (9091 ppm, n = 21, standard deviation = 1498) is comparable to reported values for modern Halimeda. Internal and cortical structures of Evgonophyllvm are exceptionally well preserved. In the basal, heavily calcified portion of the alga the medullary (interior) region of the Eugonophyllum thallus is composed of an aragonitic felt interspersed by small (20 μm diameter), parallel siphons

Facies Transitions, Depositional Processes, and Sequence Stratigraphic Relationships along a Carbonate Ramp Slope-to-Basin Profile: Mississippian Lake Valley Formation, Sacramento Mountains, Southern New Mexico

Facies Transitions, Depositional Processes, and Sequence Stratigraphic Relationships along a Carbonate Ramp Slope-to-Basin Profile: Mississippian Lake Valley Formation, Sacramento Mountains, Southern New Mexico

Persistence of the Meadow Jumping Mouse, Zapus hudsonius luteus, in New Mexico

Zapus hudsonius luteus is a subspecies of the meadow jumping mouse (Hafner et al., 1981) that is restricted to New Mexico and Arizona, where it occupies mesic habitats in lowland valleys and along montane streams (Bailey, 1913; Findley et al., 1975; Hafner et al., 1981; Hoffmeister, 1986). The subspecies has been reported from 14 localities in New Mexico (Hafner et al., 1981; R. D. Ohmart and V. C. Hink, in litt.) and 11 localities in Arizona (Hafner et al., 1981; Hall, 1981; Hoffmeister, 1986). In New Mexico, these records are from the San Juan Mountains (El Rito area), Sangre de Cristo Mountains (North Williams Lake), Jemez Mountains (upper Guadalupe River drainage), Sacramento Mountains (Rio Penasco and Silver Springs Creek), and the Rio Grande Valley (Espanola to the Bosque del Apache National Wildlife Refuge; Fig. 1). Southwestern populations of the genus Zapus were initially described by Miller (1911) as a distinct species, Zapus luteus, then as a subspecies of the western jumping mouse, Zapus princeps (Z. p. luteus), by Krutszch (1954). Based on genetic relationships between southwestern Zapus and other representatives of the genus, Hafner et al. (1981) concluded that these peripheral isolates were instead conspecific with the meadow jumping mouse, Z. hudsonius, and, therefore, considered them to be Z. h. luteus.

Microfacies Analysis of the Leonardian-Guadalupian Lower Member of the San Andres Formation in the Southern Sacramento Mountains, Otero County, New Mexico

The Rio Bonito Member of the San Andres Formation records a transgression of the northwestern shelf during the late Leonardian. Late Leonardian to Guadalupian marine carbonates exposed in the Sacramento Mountains relate a marked change from equatorial tidal flat rocks of the middle Leonardian Yeso Formation. These rocks were deposited during a worldwide sea level lowstand. The Yeso-San Andres contact, previously thought to be a gradational boundary, is here interpreted as a flooding surface resulting from the eustatic sea level rise after the lowstand. Inundation of the northwestern shelf led to deposition of the thick Andres Formation marine limestone sequence within a shallow-lagoon or shelf setting. Depositional environments are predominantly subtidal and intertidal. Microfacies include packstones of comminuted bioclasts of normal saline affinities redistributed by light currents. These shoal upward at times to Dasycladacean algal grainstones interpreted as tidal bars prograding across the lagoon or shelf. More restricted wackestones and laminated mudstones occur at the base of the section and indicate a transition from tidal flat to submerged shelf. Aggradation of sediment into the intertidal zone may have occurred cyclically during San Andres deposition. One such cycle is present over the interval exposed in the Sacramento Mountains. Intertidal rocks resemblemore » tidal flat deposits of dolostone, carbonate mudstones, and a thin tongue of quartzarenite interpeted to be Glorieta Sandstone. These rocks were previously attributed to interginguing of the Yeso with the San Andres.« less

Early Permian (Middle-Late Wolfcampian) phylloid algal/Tubiphytes bioherms and associated facies along the margin of the Orogrande Basin, Hueco Mountains, west Texas

Middle-Late Wolfcampian phylloid algal/Tubiphytes biohermal complexes have been found exposed in western outliers of the Hueco Mountains of far west Texas. Paleogeographically, the bioherms are located along the fault-controlled shelf margin between the Late Paleozoic Diablo Platform and Orogrande Basin. Although Virgilian and Early Wolfcampian phylloid algal mounds are well-known from the Hueco and Sacramento Mountains, outcropping Middle-Late Wolfcampian bioherms have not been described previously from the Orogrande Basin area.The biohermal complexes are exposed in three large outliers that lie about 3 miles west of the main Hueco Mountains and extend for about 12 miles in a north-south direction. The shelf margin complexes are in the Hueco Canyon Formation and correlate to well-bedded shelf facies in the main Hueco Mountains based on fusulinid biostratigraphy.The phylloid algal/Tubiphytes shelf margin bioherms contain an upward shallowing facies succession, which consists of, in ascending order: (1) phylloid algal wackestone-bafflestone, (2) phylloid algal bafflestone-packstone, (3) phylloid algal-fusulinid bafflestone-packstone, and (4) Tubiphytes boundstones and Tubiphytes-fusulinid-phylloid algal packstones and grainstones. Unlike some previously described Wolfcampian phylloid algal buildups, the phylloid algal mound facies in these buildups contain only rare calcisponges, heliosponges, and marine radial fibrous cements. On the crest of the southern outlier there occurs a rather different type of bioherm, which contains nodular boundstones that are composed of encrusting red algae and bryozoans, and in which calcisponges are common. That bioherm is thought to be slightly younger in age than the phylloid algal/Tubiphytes bioherms, and it might also have been formed in a deeper-water setting.Bordering the phylloid algal/Tubiphytes bioherms on the seaward side are overlapping tongues and channels of lithoclastic-skeletal debris and skeletal grainstones and packstones. Some of these forereef units extend seaward into slope facies, which consist of dark-gray cherty limestones that generally lack skeletal fossils, but contain a rich ichnofossil assemblage in the shallower upper slope beds. Backreef facies consist mainly of skeletal-peloidal packstones and wackstones.The Hueco Mountains outlier exposures are significant because: (1) they establish the presence of a Middle-Late Wolfcampian shelf margin with distinct topographic relief in the southern Orogrande Basin, and (2) they provide an easily accessible field laboratory to study Middle-Late Wolfcampian shelf-to-basin facies relationships and shelf margin bioherms. Middle-Late Wolfcampian shelf margin bioherms are of particular interest because they represent an important transitional stage in the evolutionary history of Late Paleozoic reef communities, and because they form important petroleum reservoirs just to the east in the Delaware and Midland Basins.

Surface Gamma-Ray Logs as a Correlation Tool Between Outcrop and Subsurface: Application to the Silurian-Devonian of West Texas and Southern New Mexico

Outcrop gamma-ray logs are an excellent tool for correlation between surface measured sections and subsurface well logs. The work presented here illustrates the utility of constructing such profiles and the applicability of this technique to carbonate sequences such as those of the Permian basin. Outcrop sections with gamma-ray profiles have been measured over the Silurian-Devonian section in three separate areas. These sections are located in the Sacramento Mountains of southern New Mexico, and the northern Franklin Mountains and Hueco Mountains of west Texas. A hand-held Scintrex Model BGS-4 Digital Scintillometer was used to measure surface gamma radiation while detailed stratigraphic sections were being measured and described. Data were collected at regularly spaced intervals along the section. The scintillometer detects natural radiation emitted by radioactive elements that occur in most clay minerals and generally are more abundant in shales than in sandstones or carbonates. The lithology of poorly exposed or covered units also may be inferred from surface gamma-ray profiles. Organic-rich black shales are particularly radioactive, as are condensed sections. The strength of this method does not lie in the absolute reading of gamma radiation. The value of this tool lies in recognizing patterns within each profile, directly relating these patternsmore » to their associated facies, and correlating them with subsurface profiles.« less

An axial view of a metamorphic core complex: Crustal structure of the Whipple and Chemehuevi Mountains, southeastern California

A 135‐km‐long, NW‐SE trending, seismic refraction/wide‐angle reflection profile provides a unique along‐strike view of the crustal structure of a belt of metamorphic core complexes in southeastern California: the Whipple, Chemehuevi, and Sacramento mountains metamorphic core complexes. Interpretation of the seismic data was done by two‐dimensional forward modeling of travel times and amplitudes. The final model consists of (1) a thin (< 1.5 km) veneer of upper plate and fractured lower plate rocks (velocities of 1.5–5.3 km s−1) overlying a fairly homogeneous basement with velocities of 6.0 km s−1; (2) a localized, high‐velocity (6.4 km s−1) body, situated directly beneath the Whipple Mountains; (3) a 6.3–6.4 km s−1 middle crust that is thickest beneath the core complexes; (4) a 6.65±0.15 km s−1 lower crust; (5) crustal thickness of 27 km with a deeper crustal root (3 km) beneath the Whipple Mountains metamorphic core complex; and (6) a Pn velocity of 8.0±0.10 km s−1. The crustal structure that underlies the belt of metamorphic core complexes provides new insights into the processes that control extension in the deep crust. Upper crustal velocities are higher beneath the Whipple Mountains (where velocities increase to 6.4 km s−1 at ∼5 km depth) than beneath the Chemehuevi and Sacramento mountains. In addition, midcrustal discontinuities rise 2–5 km beneath the Whipple complex compared to the other complexes. These observations support greater uplift and a slightly deeper midcrustal origin for the rocks now exposed in the core of the Whipple Mountains compared to rocks in the Chemehuevi and Sacramento mountains. Despite the enhanced uplift and extension in the Whipple Mountains, the crust is thicker here (30 km) than anywhere else along the Colorado River extensional corridor. This may be in part a relic of compressional and magmatic thickening during the Mesozoic. However, we suggest that inflation of the crust during Tertiary extension was the dominant mechanism. Both mantle‐derived magmatism and lateral ductile inflow in the crust are proposed.