Mural Limestone Research Articles

Lower Cretaceous (Aptian-Albian) Bisbee Group, Arizpe area, northern Sonora, Mexico: Integrated biostratigraphy, age and provenance from U-Pb and Hf isotopes

New data on the stratigraphy, biostratigraphy, detrital zircon U-Pb, and Hf isotope geochronology of a Lower Cretaceous succession of the Bisbee Group in northern Sonora, Mexico that accumulated in the Altar-Cucurpe sub-basin help to constrain ages of their formations and documents the detrital provenance. These tectonically shortened strata, studied in the Arizpe area, crop out in the Cerro La Ceja block (CCB) composed of the upper Morita Formation, the complete Mural Limestone, and Cintura Formations, and the lower part of La Juana Formation. The Sierra Los Azulitos block (SAB), composed of the Mural Limestone, is thrust eastward over the CCB. A measured section of the Mural Limestone in the SAB block includes the upper Tuape Shale, Los Coyotes, Cerro La Puerta, and part of the Cerro La Espina members whose age is constrained as upper Aptian to lower Albian based on benthic and planktonic foraminifera, bivalves and echinoderms. The age of the boundary between the Morita and Tuape Shale Member is constrained by detrital zircon maximum depositional ages (MDA) of 115.8 ± 1.1 Ma and 113.9 ± 0.8 Ma, respectively. The MDA of the upper Mural is 111.6 ± 1.6 Ma and the MDA of basal Cintura Formation is 108.8 ± 1.1 Ma. The La Juana Formation has upper Albian bivalves and echinoderms.
 About one-quarter of the detrital zircon grains dated from the five sandstone samples have Proterozoic ages with age peaks that clearly indicate provenance from basement rocks of southwestern Laurentia. More than two-thirds of the grains are of Jurassic and Early Cretaceous age in about the same proportions, and have main peaks at 166, 150, and 118 Ma. The Jurassic grains may have sources in rocks of the Jurassic Cordilleran continental magmatic arc of southwestern North America, and the most probable sources for the Cretaceous grains are arcs of the Peninsular Ranges batholith that by that time were located adjacent to coastal Sonora. Jurassic and the Early Cretaceous zircons dated from 195.3 to 106.8 Ma both have mixed εHf(t) values that range from 9.53 to -21.28, and TDM model ages between 0.4 to 0.8 Ga for the primitive grains, and 0.8 to 1.3 Ga for the more evolved zircons. The probable source for the Jurassic grains with negative εHf values might be local granites of that age in northern Sonora that have Neoproterozoic TDM model ages. In contrast, sources for the Jurassic zircon with positive Hf values may be Jurassic igneous and metaigneous rocks of the Peninsular Ranges Batholith of the Californias, although data reported are scarce. Similarly, the Early Cretaceous detrital zircon grains with mixed positive and negative εHf values may have provenance from igneous rocks of the Peninsular Ranges Batholith-Sierra Nevada magmatic arc.

Aptian–Albian sequence stratigraphy, biostratigraphy, chemostratigraphy, and chronostratigraphy: Sonoran shelf and Tamaulipas Basin, Mexico

The Late Jurassic extensional Bisbee Basin in northwestern Mexico and southwestern United States lay between the Pacific magmatic arc and subduction zone and the cratonic Comanche shelf. Aptian to late Albian fluvial, deltaic, and shallow marine sediments and patch reefs of the Bisbee Group filled the basin. The upper Barremian(?)–lower Aptian Cerro de Oro Formation and its equivalent Rancho La Colgada Formation are overlain by the Morita, Mural, Cintura, and La Juana formations. The paralic Morita Formation and the marine Mural Limestone grade southeast into the marine Agua Salada and Lampazos formations, which correlate with deep-water facies in the Chihuahua trough and Tamaulipas Basin. Three late Aptian to middle Albian long-term depositional cycles were approximately 2.5 myr in duration.Foraminifers, calcareous algae, bivalves, rudists, corals, dinoflagellates and scarce ammonites compose the Mural Limestone biostratigraphy. First-occurrence datums of Aptian–Albian benthic Foraminifera are integrated with planktonic Foraminifera and rudists and define biostratigraphic zones that correlate with the Comanche shelf. In the Chihuahua trough and Tamaulipas Basin planktonic Foraminifera and nannofossils identify global zones. Stage boundaries on the Sonora shelf are correlated by a comprehensive biostratigraphic database.Carbon isotope chemozones correlate with Tethyan chemostratigraphy. The lower Aptian OAE 1a and uppermost Aptian–lowest Albian OAE1b chemozone are in the Lampazos section. The negative δ13C excursions and fossil evidence correlate with global Jacob, Kilian, Paquier, and Leenhardt levels. Low-oxygen water masses flooded the shallow shelf and even affected coral-rudist buildups.

Depositional environments and transgression-regression cycles in the Mural Limestone, central Sonora, Mexico

Results of analytical studies in the Mural Limestone of the Bisbee Group exposed in northwest Mexico, carried out to determine the paleo-bathymetry of the marine depositional environment, reveal that deposition occurred in a restricted carbonate platform environment, varying from inner-neritic to shelf margin with the development of lagoon, shoal and biohermal conditions, reflecting minor transgression-regression cycles. Furthermore, the fossiliferous content and lithological characteristics support the interpretation that the studied section corresponds to the transition between Cerro La Puerta and Cerro La Espina members of the Mural Limestone.

Depositional Environments and Transgression-Regression Cycles in the Mural Limestone, Central Sonora, Mexico

Depositional Environments and Transgression-Regression Cycles in the Mural Limestone, Central Sonora, Mexico

Paleoecology of a Lower Cretaceous (lower Albian) marine community of Cerro de Oro, central Sonora, Mexico

The Early Cretaceous time is characterized by regression and transgression events in the oceans, which impulsed the radiation processes of species. In particular, these oscillation events controlled the distribution of marine invertebrates because they generated geographical barriers and the geographical connection between the different ecological niches. These events promote speciation of which there is a limited record in the state of Sonora. This work is the first quantitative paleoecological study of Early Cretaceous marine invertebrate fauna from central Sonora. The Mural Limestone exposed at the El Caloso Hill, located in the Cerro de Oro area, comprises 28 invertebrate taxa belonging to several groups such as corals, bivalves and gastropods, cephalopods, and echinoderms. These paleocommunities are part of the Early Cretaceous Mexican Sea, which covered the southwestern United States and Mexico during regressive/transgressive events. To evaluate and compare the diversity and richness of these paleocommunities, paleoecological indexes like the Simpson Dominance and Evenness index, Shannon index, Menhinick Species Richness, Total Chao Richness, and Pielou's Evenness index have been applied. In addition, the distribution described for these paleocommunities was defined as a clumped type. The limestone textures described for these environments comprises wackestone, and floatstone, typical of a shallow lagoon environment with low energy. Finally, the dominant guilds were represented by epifaunal sessile suspension, infaunal sessile suspension, and semi-infaunal facultative vagile suspension feeders.

Chemostratigraphy of the lower Cretaceous Mural Limestone, Rancho Bufalo section, Sonora, Mexico: Implications for OAE 1b

A multidisciplinary approach of the Aptian-Albian Mural Limestone including petrography, geochemistry and stable isotope data is undertaken for the 660-m thick Rancho Bufalo section of northeastern Sonora to characterize source of rare earth elements and to infer paleo-redox conditions. The lithofacies of limestones range from mudstone to boundstone. The unaltered signatures of the δ13C values are confirmed by using Mn and Sr concentrations and their ratios along with the absence of correlation between δ13C and δ18O values. The limestones have large variations in ΣREE and Al2O3 contents (8.0 − 95.8 ppm; 0.45% − 7.10 ppm; respectively). Most limestones show non-seawater-like REE + Y patterns with positive Eu anomalies and negative Ce anomalies. The limestones have positive Ba anomalies in six intervals and most of them were resulted due to weathering and depositional processes and not due to sea level variations. The redox proxies of U, V/Cr and V/(V + Ni) indicate that the bottom water conditions fluctuated from oxic to anoxic. The high-resolution carbon isotope curve reveals four OAE 1 b sub-events: Jacob, Kilian, Paquier, and Leenhardt. The carbon isotope curve of the Rancho Bufalo section is comparable to that of the Cerro Pimas and Peregrina Canyon sections of Mexico.

Facies and sequence stratigraphic architecture of the Mural Limestone (Albian), Arizona: Carbonate response to global and local factors and implications for reservoir characterization

AbstractThis study documents the detailed facies and sequence stratigraphic architecture of a multi‐cyclic patch‐reef and its associated ramp interior facies that formed during Oceanic Anoxic Event 1b in the Mural Limestone, Arizona, USA. Ramp interior facies are comprised of bedded wackestone/packstone, rudist build‐up and coral–algal patch‐reef facies located north of Bisbee, Arizona, at the Grassy Hill locality. The larger multi‐cyclic patch‐reef that developed coevallyca5 km to the south of Grassy Hill consists of a high‐angle windward margin with a narrowca70 m long reef frame containing vertically zonatedMicrosolena, Actinastrea, diverse branching coral and rudist assemblages, and an 870 m long low‐angle leeward margin comprised of reef debris rudstone and grainstone shoal facies. Similar reef geomorphology and orientation is documented across the Gulf of Mexico and reflects the shelf‐wide north to north‐east‐trending prevailing wind and current energies. Controls affecting reef formation and growth patterns include changes in accommodation space associated with low‐amplitude global sea‐level rise and regional thermotectonic subsidence, local accommodation space and nutrient fluctuations associated with the inner shelf depositional setting within a humid and siliciclastic‐rich environment. Four aggradational to retrogradational high‐frequency sequences are documented in Arizona: High‐frequency sequences 1 and 2 represent the first pulse of patch‐reef development in an overall second‐order marine transgression over the Sonora/Bisbee Shelf. These sequences correlate to δ13C signatures associated with Oceanic Anoxic Event 1b across the Gulf of Mexico and suggest that carbonate reefs persisted on the ramp interior during this time. High‐frequency sequences 3 and 4 record a second brief transgression and backstepping of reef facies followed by the final regression of shallow shelf carbonates that correlates to more robust patch‐reef development in Sonora, Mexico. The patch‐reef at Paul Spur is an excellent outcrop analogue for productive patch‐reefs in the Maverick Basin (Comanche Shelf) of Texas. Detailed facies mapping of this outcrop analogue shows that the greatest reservoir potential is contained within the backreef grainstone shoals where primary porosity of up to 15% is observed.

Stratigraphy, detrital zircon geochronology and provenance of the Morita formation (Bisbee Group) in northeastern Sonora, Mexico

The Morita Formation is a Lower Cretaceous unit of the Bisbee Group that crops out in northern Sonora and southeastern Arizona where it was deposited within the Altar-Cucurpe, and Huachuca sub-basins of the Bisbee basin, respectively. In northern Sonora it either overlies strata of the Upper Jurassic Cucurpe Formation, the Glance Conglomerate, or strata of the Lower Cretaceous Cerro de Oro/Rancho La Colgada formations, and it grades upward into the Mural Limestone. In this contribution we characterize the stratigraphy, petrography and detrital zircon geochronology of the Morita Formation along a ca. 60 km-long northwest-southeast transect that includes the complete columns of the Mule Mountains in southeastern Arizona and the sierras San José, Anibacachi and Rancho Búfalo area in northeastern Sonora. Thickness of these sections varies from a minimum of 555 m in Sierra Anibacachi to a maximum of 855 m in Rancho Búfalo. In most sections, the lower half of the Morita consists of reddish siltstone and mudstone with pedogenic calcareous nodules with interbeds of mostly single-storey sandstone bodies, which changes to isolated multi-storey sandstone bodies in its upper half. In contrast, the lower part of the Rancho Búfalo column consists of reddish mudstone/siltstone with isolated, fining-upward, clast-supported, fine-pebble conglomerate beds that grades into reddish to purple, brown and green mudstone/siltstone with isolated, single-storey to multi-storey sandstone beds of its upper part. Lithofacies association in the Morita Formation suggest it is of fluvial origin but ichnofossils and local herringbone- and flaser-cross bedding in its upper part indicate intermitent marginal marine sedimentation preceding the transgression that deposited the overlying Mural shelf.Sandstone composition of the Mule and San José mountains is dominantly feldspatho-quartzose indicating dominant provenance from basement uplift and transitional continental areas, while composition at Sierra Anibacachi and Rancho Búfalo ranges from feldspatho litho-quartzose to litho-quartzose, indicating provenance from recycled orogen. Detrital zircon grains dated from samples of all the studied sections share very similar populations of Proterozoic to Mesozoic ages, but the Mesoproteozoic and Triassic grains compose ca. 64% of the total grains. Detrital zircon grains of Lower Cretaceous age that are only present in samples from the bottom of the Mule and San José Mountains help to constrain a maximum depositional age of 125.5 ± 0.8 Ma for inception of sedimentation of the Morita Formation in this region. According to this age and to observed stratigraphic relationships, we interpret that the lower contact of the Morita Formation is a disconformity with the underlying Glance Conglomerate, or with the Cerro de Oro Formation in northeastern Sonora. Similarly, according to regional stratigraphic relationships and age data known from the Cucurpe-Tuape region, in north-central Sonora, the age of this unit may be constrained to the Aptian stage, in Sonora, from ca. 125 to 115 Ma. Regional isopach curves confirm merging of the sub-basins in northeastern Sonora, continuous sedimentation of the Morita Formation, and presence of the Cananea high separating them.

Detrital zircon record of Mesozoic volcanic arcs in the Lower Cretaceous Mural Limestone, northwestern Mexico

The northwestern part of Mexico, southern Arizona, and southern California witnessed extensive arc magmatism during Late Triassic to Early Cretaceous. Here, we present results from petrography, whole‐rock geochemistry (major and trace including rare‐earth elements), and detrital zircon geochronological studies that were carried out on sandstones and shales of the Mural Limestone exposed in the Rancho Bufalo area. Based on the petrography, sandstones are classified as sublitharenite, litharenite, feldspathic litharenite, and lithic arkose. The sandstones contain higher proportion of monocrystalline quartz than polycrystalline quartz. Sandstones and shales from the various members of the Mural Limestone show large variations in major and trace elements concentrations. The average concentrations of ΣREE (total ΣREE) are lower in sandstones than in shales. The chondrite‐normalized REE patterns of the sandstones and shales are moderately fractionated with light REE (LREE) enrichment relative to heavy REE (HREE) and with flat to slightly depleted HREE patterns. The chemical index of alteration values and A–CN–K relationships of sandstones indicate low to moderate intensity of weathering in the source region; however, shale samples suggesting the source rocks were subjected to moderate intensity of chemical weathering. The chondrite‐normalized REE patterns, Al2O3/TiO2ratio, elemental ratios like Cr/V, Y/Ni, and Eu‐anomaly (Eu/Eu*), and bivariate plots suggest that the sandstones and shales were derived mainly from felsic source rocks. Detrital zircon U–Pb geochronology of sandstones from Fronteras, Rancho Bufalo, and Cerro La Ceja members of the Mural Limestone show five main groups of detrital zircon age populations: (a) Proterozoic, (b) Palaeozoic, (c) Triassic, (d) Jurassic, and (e) Early Cretaceous. Our study suggests that continental volcanic arcs in northwestern Mexico, southern Arizona, and southern California served as the provenance for the Late Triassic‐Jurassic detritus, whereas the Early Cretaceous zircon grains were derived mainly from the active Alisitos volcanic arc along the northwestern coast of Mexico.

Aptian and Albian (Early Cretaceous) ammonites from Lampazos and the Bisbee groups (Sonora State, northwest Mexico)

A taxonomic study was carried out of 34 ammonites collected from the Agua Salada and Nogal formations (Lampazos Group) and the Mural Limestone (Bisbee Group) in the Sonora State, northwest of Mexico. Twenty-one taxa are described, with 13 being endemic of the Central Atlantic province: Puzosia (Anapuzosia) sp. juv. cf. tucuyensis, Parasilesites cf. bullatus, Beudanticeras parandieri, "Beudanticeras" newtoni, Beudanticeras sp., Desmoceras (Pseudouhligella) cf. vetus (en=endemic), Engonoceras complicatum (en), Engonoceras stolleyi, Protengonoceras gabbi (en), Protengonoceras neolobitoides (en), “Eodouvilleiceras cf. adkinsi” (en), Huastecoceras? sp. (en), Hypacanthoplites sp., Immunitoceras immunitum (en), Immunitoceras cf. trinitensis (en), Kazanskyella minima (en), Kazanskyella spathi (en), Quitmanites ceratitosus (en), Quitmanites fosteri (en), and Quitmanites sp. (en), between them we erected one new species: Protengonoceras neolobitoides. The record of Aptian-Albian ammonites in Sonora is scarce, therefore the ammonites herein studied are valuable due to the number of endemic taxa that could be used in future biostratigraphic works. In addition, these taxa allow a revision of the stratigraphic frame: The Agua Salada Formation is now considered from late Aptian to early Albian; the Nogal Formation is confined to late Albian; the Cerro Las Conchas succession is limited to middle Albian; and the age of the Cerro La Ceja Member is late Aptian.

Stratigraphy and sedimentology of the upper Aptian–upper Albian Mural Limestone (Bisbee Group) in northern Sonora, Mexic

The Cretaceous Mural Limestone in north-central Sonora, Mexico, is similar in stratigraphic succession and lithofacies to correlative strata of the Bisbee Group in the southwestern USA. It consists of 800–900 m of shallow-marine limestone and shale that range in age from late Aptian to mid-late Albian. Carbonate lithologies generally consist of oyster-bearing wackestone–packstone and rudistid boundstone, the latter forming discontinuous, mound-like bioherms. Siliciclastic intervals are dominated by dark gray to black shale with ammonoids and trigoniids. The Mural Limestone in north-central Sonora consists of six formal members, defined here. In ascending order, these are the Cerro La Ceja, Tuape Shale, Los Coyotes, Cerro La Puerta, Cerro La Espina and Mesa Quemada members. Based on ammonoids, trigoniids, and benthic foraminifers, the lower two members are late Aptian and the upper four members range from latest Aptian–mid-late Albian. The marine strata of the Mural Limestone separate two continental red bed successions that are difficult to differentiate on the basis of lithology and so provide an important marker in the Lower Cretaceous section of Sonora. Consistent stratigraphic successions throughout northern Sonora correlate with similar successions in the southwestern USA and northeastern Mexico, well beyond the limits of the Bisbee Basin, suggesting that global eustasy played an important role in deposition of these strata. Unusually thick Aptian shales of the Mural Limestone probably accumulated during post-rift tectonic subsidence, which enhanced the thickness of these marine-shelfal deposits. Consistent, predictable stratigraphy in the Mural Limestone across northern Sonora and the USA–Mexico border region likewise indicates that depositional models involving deep-marine gravitational emplacement of shallow-water facies proposed by some workers are untenable.

Lower Cretaceous corals and stratigraphy of the Bisbee Group (Cerro de Oro and Lampazos areas), Sonora, Mexico

Stratigraphic sections of the Bisbee Group in northwestern Mexico have been studied. Several lithostratigraphic units from the Cerro de Oro and Lampazos areas, characterized by mainly carbonate/clastic shallow water sequences, are discussed in this paper. In the Lampazos area these sediments are grouped into the Agua Salada, Lampazos, Espinazo del Diablo, and Nogal Formations. In the Lower Cretaceous sections of the Cerro de Oro area, the Cerro de Oro, Morita, and Cintura Formations, as well as the Mural Limestone are distinguished. Scleractinian corals are reported from Upper Barremian to Middle Albian reefal limestone intervals within the Cerro de Oro, Espinazo del Diablo and Nogal Formations, and within sequences of the Mural Limestone. They comprise 32 genera belonging to seven suborders. Six new species are described: Columastrea paucipaliformis n. sp., Columnocoenia minima n. sp., Keriophyllia roniewiczae n. sp., Cladophyllia mexicana n. sp., Pleurostylina major n. sp., and Stiboriopsis sonoraensis n. sp.

Mass-gravity deposits and structures in the Lower Cretaceous of Sonora, Mexico

Research Article| December 01, 1998 Mass-gravity deposits and structures in the Lower Cretaceous of Sonora, Mexico Mary Beth McKee; Mary Beth McKee 17118 Clow Road, Winneconne, Wisconsin 54986 Search for other works by this author on: GSW Google Scholar Thomas H. Anderson Thomas H. Anderson 2University of Pittsburgh, Department of Geology and Planetary Science, 321 Old Engineering Hall, Pittsburgh, Pennsylvania 15260 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1998) 110 (12): 1516–1529. https://doi.org/10.1130/0016-7606(1998)110<1516:MGDASI>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Mary Beth McKee, Thomas H. Anderson; Mass-gravity deposits and structures in the Lower Cretaceous of Sonora, Mexico. GSA Bulletin 1998;; 110 (12): 1516–1529. doi: https://doi.org/10.1130/0016-7606(1998)110<1516:MGDASI>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Rocks that form the south flank of Sierra Azul, northern Sonora, Mexico, are correlative with the Jurassic(?) and Lower Cretaceous Bisbee Group of southern Arizona. We interpret them as basinal marine deposits featuring both mass-gravity deposition and deformation. Thick (hundreds of meters) bodies of sediment showing some internal disaggregation are separated by slide surfaces that cut down-section into footwalls. We believe that further mass movement formed these stacked slide masses into northwest-trending, southwest-vergent folds, though they contain many features normally associated with direct tectonism. Thus, the folds are not the product of Laramide crustal shortening, but rather they reflect earlier (Jurassic and Cretaceous) high-angle crustal movement that produced the paleo-upland (the Cananea high), the basin, and the slope that guided their development. We suggest their vergence shows paleoslope, not tectonic transport.This basinal marine sequence contains blocks of reefal limestone equivalent to the upper Mural Limestone (Bisbee Group) that have been widely used as evidence of an in situ carbonate bank; here, they are allochthonous. We suggest that they may have been derived from the margin waters of the Cananea high. Confusion of structures such as these formed by mass-gravity processes may be one reason that the pattern of Laramide deformation in northern Sonora is still poorly defined. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.

Facies architecture of a Lower Cretaceous coral-rudist patch reef, Arizona

The Lower Cretaceous Mural Limestone marks the maximum marine incursion into southeast Arizona during Aptian-Albian time and records the middle Cretaceous transition from coral-dominated to rudist-bivalve dominated reefs. Upper Mural Limestone facies are most often dominated by corals. However, rudists form significant frameworks at some localities, one of which is described in this paper. The paleoenvironmental distribution of three potential reef-builders (corals, rudists, and ‘oysters’) were studied at this patch reef locality. Corals built the framework of the inner reef core. The rudist Petalodontia initially gained a foothold in sheltered areas among corals and subsequently built a framework in the outer reef core. Caprinid rudists formed mounds in the outer reef to back reef areas. The rudists Toucasia and Monopleura and the oyster-like bivalve Chondrodonta formed beds or were scattered in the reef-flank and shelf lagoon sediments and did not contribute to the reef framework. Upper Mural Limestone reefs are important examples of the coexistence of corals and rudists during this middle Cretaceous faunal transition period. This study supports the idea that rudist-bivalves initially colonized protected back-reef areas early in the Cretaceous and only later in the Cretaceous did rudists dominate reef frameworks.

Bisbee Basin and Its Bearing on Late Mesozoic Paleogeographic and Paleotectonic Relations Between Cordilleran and Caribbean Regions: ABSTRACT

The Bisbee Group and its correlatives in southern Arizona, the New Mexico panhandle, and adjacent parts of Mexico are composed dominantly of Lower Cretaceous nonmarine, marginal marine, and shallow marine deposits, but apparently also include upper Upper Jurassic and lower Upper Cretaceous strata. Farther west, in southwestern Arizona and southeastern California, lithologically similar nonmarine strata of the McCoy Mountains Formation and its correlatives occupy the same general stratigraphic position as the Bisbee Group, but are poorly dated and may be older. The rifted Bisbee basin was a northwestern extension of the Chihuahua Trough, a late Mesozoic arm of the Gulf of Mexico depression. Basal zones of the Bisbee Group were deposited as alluvial fans marginal to active fault blocks during the rift phase of basin development, and are intercalated locally with lavas and ignimbrites. Subsequent thermotectonic subsidence of thinned crust beneath the Bisbee basin allowed intertonguing fluvial, lacustrine, deltaic, strandline, and marine shelf facies to invade the basin and bury the foundered fault-block topography. The upper Lower Cretaceous (Aptian-Albian) Mural Limestone was deposited during the phase of maximum transgression. Differing sandstone petrofacies of quartzose, arkosic, and volcaniclastic character reflect derivation of clastic detritus from varied sources bordering the Bisbee basin.more » The Bisbee basin and Chihuahua Trough developed in relation to Jurassic opening of the Gulf of Mexico, and were associated with changing plate configurations and motions throughout the Mesoamerican region.« less

Facies Development in a Lower Cretaceous Coragrudist Patch Reef (Mural Limestone, Southeast Arizona)

Patch reefs within the Mural Limestone (Bisbee Group, Cretaceous, S. E. Arizona) are dominated by scleractinian corals, algal stromatolites, and rudist bivalves. Reef development occurred during the Early Albian transgression into the Chihuahua Trough, a north-west extension of the Tethyan seaway. One patch reef (270 m x 120 m) was studied in detail. Species abundance and growth form were estimated in each of 57 quadrats of l m2 along ten transects. Peels and slabs were used for detailed identification and for lithologic determination. Facies development is controlled by wave energy. The crescent-shaped front is dominated by branching and mas­ sive corals. Behind this fades a zone of laminar Microsolena (coral) along with algal stromatolites form the crest facies. Rudists dominate the back reef. The Petalodontia subfacies is closest to the reef crest and consists of thickets of gregarious, thick-walled Petalodontia, a few corals, and interbedded molluscan debris. The caprinid subfacies is farthest from the reef crest and contains corals, bivalves, and molluscan fragments. A debris facies of molluscan fragments and foraminifera surrounds the reef. Reefs of roughly correlative age in Texas are dominated by rudists, not corals. The Texas rudist frameworks are often associated with evidence of abnormal salinity conditions which were presumably intolerable for corals.

Dolomite and Dedolomite in Mural Limestone, Lower Cretaceous, Arizona and Sonora: ABSTRACT

End_Page 800------------------------------Three petrographically distinct types of dolomite occur in the upper member of the Mural Limestone, lower Albian, of southeastern Arizona and northeastern Sonora: (1) very fine to medium grained sucrosic dolomite occurs replacing lime mud fills of constructional reef cavities 5 cm to 2 m in diameter, emplaced along stylolites and in irregular patches of pressure-solved matrix in rudstones and floatstones, and replacing matrix in oolitic and skeletal packstones; (2) medium to extremely coarse ferroan baroque dolomite, characterized by curved cleavage and sweeping extinction, is an important void filling cement; and (3) euhedral, non-ferroan dolomite cement partly fills some late fractures. Available evidence indicates that dolomitization occurred in four episodes. Petrographic and field evidence suggests that sucrosic dolomite in reef cavity fills and in coarse packstones preceded baroque dolomite cement. Baroque dolomite followed calcite rim cements and some blocky spar, and was followed by additional blocky spar. Fractures containing non-ferroan dolomite cement postdate all of these cements. Pressure solution-related dolomite formed still later, during middle to late burial diagenesis and tectonism. Dedolomitization affected all dolomite types in the Mural Limestone. Nearly all sucrosic dolomite has been calcitized. The resultant fabric consists of calcite rhombs in a mass of anhedral calcite, stained by exsolved iron. Baroque dolomite cement is commonly partly calcitized. It alters to single crystals of rusty calcite in optical continuity with the dolomite, retaining the latter's curved cleavage and sweeping extinction. Dedolomitization in the Mural is attributed to exposure to low-Mg fresh water under near-surface temperatures and pressures. These conditions have probably been in effect since the mid-Tertiary. End_of_Article - Last_Page 801------------

Depositional Model of Early Cretaceous Coral-Algal-Rudist Reefs, Arizona

Patch reefs composed mainly of corals and stromatolites are well developed in the Lower Cretaceous Mural Limestone of southeastern Arizona. This biota is similar to that forming buildups in the downdip Lower Cretaceous sections of the Gulf Coast at Fairway and Stuart City fields in Texas and the Golden Lane-Poza Rica field in Mexico. In contrast, updip Lower Cretaceous buildups in central Texas consist mainly of rudists, and corals are rare. The reefs of the Mural Limestone are up to 25 m thick and about 1.5 km long. They grew during the early Albian transgression from the Chihuahua trough into Arizona. The reef-core and flank facies are coral-stromatolite-rudist boundstone and coral-rudist fragment packstone. The reef-core facies is dominated by the colonial corals Actinastrea and Microsolena. The latter coral commonly is encrusted by stromatolites. Caprinids and thick-walled monopleurids are secondary in abundance. Associated perireef facies are peloid-ooid grainstone, mollusk-miliolid-orbitolinid wackestone, and ostracod-mollusk-skeletal algal wackestone and sandstone-shale. The perireef shallow shelf is characterized by Monopleura, Chondrodonta, Toucasia, and nerineids. The nearshore restricted lagoon and shoreface to tidal-flat environments were occupied by Exogyra and Crassostrea communities and the Arenicolites association. Where megafossils are absent, the shelf environment is characterized by assemblages of miliolids, Orbitolina, and agglutinated forams. The distribution of algal types is also closely related to environments. These facies and communities have predictable stratigraphic relations.

Lower Cretaceous Carbonate Shelf in Southeastern Arizona and Northeastern Sonora, Mexico: ABSTRACT

The Mural Limestone (Bisbee Group, Lower Cretaceous) in southeast Arizona and northeastern Sonora exposes a broad carbonate shelf and an irregular, migrating shelf margin at the northwest end of the Chihuahua trough. The upper Mural represents the culmination and initial regressive phases of an Aptian-Albian transgression. It is underlain by nearshore clastic strata and limestones of the lower Mural and the Morita Formations, and overlain by clastic beds of the Cintura Formation. Study of the narrow outcrop belt from the Mule Mountains in Arizona to where the Mural disappears beneath Quaternary volcanics, 70 km to the south in Sonora, reveals an overall pattern of southward-deepening water. The upper Mural thickens from 50 to nearly 300 m over this distance. Facies presen in Arizona are, from north to south; (a) shallow lagoonal packstones and wackestones; (b) a broad oolite and pelletoid-sand shoal; and (c) a muddy, open shelf with small, isolated reefs in waters at least 10 m deep. In Mexico, End_Page 547------------------------------ larger isolated reefs give way southward to reef banks 5 to 8 km across, surrounded by mudstones and wackestones containing pelagic microfossils. Shallow-water deposits of the southernmost bank step progressively southeastward over deeper water limestones. Reef cores in most of this belt consist of platy, branching megacolonies of the coral Microsolena, encrusted by thick, laminated stromatolites. In the north, rudists are abundant only on reef flanks and caps, and in shallow interreef and backreef areas. Rudists increase in abundance on reef banks to the south, as do various encrusting and head-forming algae. Thoroughly bound rudist-coral-algal frames dominate the southernmost reefs. Lagoonal limestones of the early regressive phase overlie the reef interval, and are punctuated in the south by the thick sandstone and shale wedge. Nearshore and continental clastic units of the Cintura Formation ended carbonate deposition in this area. End_of_Article - Last_Page 548------------

Some Early Cretaceous Calcareous Algae from Cochise County, Arizona

An early Cretaceous marine algal floral consisting of Boueina pygmaea Pia, Neomeris pfenderae, n. sp., Arcicularia americana, n. sp., and Archaeolithothamnium? sp. is described from the Mural Limestone of Cochise County. A study of known occurrences of Neomeris suggests that Recent species of this genus are and Cretaceous species were, restricted to tropical marine environments (isocrymes above 20 degrees C.) within the lower littoral and upper infralittoral zones (O to 10 m.).