Shallow Shelf Facies Research Articles

Differential sedimentary evolution of typical aulacogens of Meso-Neoproterozoic in North China craton

Many countries and regions in the world have obtained industrial oil flow in the Meso-Neoproterozoic sedimentary strata and formed commercial exploitation in recent years. The development horizon of high-quality source rocks in the Proterozoic in North China can be compared with the international, indicating that the Meso-Neoproterozoic in North China has great exploration potential. The sedimentary characteristics of typical aulacogen in multiple cratons in the Meso-Neoproterozoic North China Craton are compared and studied by using field outcrop data, drilling data and analysis and test data, aiming to provide sedimentary support for the prediction of oil and gas distribution and evaluation of exploration field in the Meso-Neoproterozoic in this area. The results show that there are four sedimentary systems in the study area, including Marine clastic rock sedimentary system, Marine carbonate sedimentary system, Marine-continental transitional facies sedimentary system and glacial sedimentary system. They are divided into seven sedimentary facies types: barrier coastal facies, non-barrier coast facies, shallow shelf facies, carbonate platform facies, reef facies, fan delta facies and glacial facies, and further divided into 15 subfacies and 21 microfacies. On this basis, the Meso-Neoproterozoic sedimentary filling sequences of two typical aulacogens, Yanliao and Xiong 'er, in the study area are clarified, showing that the formation time of each sedimentary filling sequence stage of different aulacogens is different, and the rock characteristics, lithology combination, lithologic structure, contact relationship, vertical sequence and sedimentary facies assemblage of the same sedimentary filling sequence stage are obviously different. The filling characteristics of the two aulacogens completely record the geological events related to the breakup of the Colombian supercontinent.

Microfacies types and distribution of epicontinental shale: A case study of the Wufeng–Longmaxi shale in southern Sichuan Basin, China

For black shales, laminae and bedding are hard to identify, grain size is difficult to measure, and trace fossils do not exist. Taking the Ordovician Wufeng – Silurian Longmaxi shale in southern Sichuan Basin, China, as an example, the types, characteristics and models of microfacies in epicontinental shale are analyzed by means of full-scale observation of large thin sections, argon-ion polishing field emission-scanning electron microscopy (FE-SEM), and kerogen microscopy. The epicontinental sea develops delta, tidal flat and shelf facies, with black shale found in microfacies such as the underwater distributary channel and interdistributary bay under delta front facies, the calcareous and clayey flats under intertidal flat facies, the calcareous and clayey shelfs under shallow shelf facies, the deep slope, deep plain and deep depression under deep shelf facies, and the overflow under gravity flow facies. Basinward, silty lamina decreases and clayey lamina increases, the grain size changes from coarse silt to fine mud, the silica content increases from about 20% to above 55%, the carbonate and clay minerals content decreases from above 40% to around 10%, and the kerogen type changes from type II2 to type II1 and type I. Provenance and topography dominate the types and distribution of shale microfacies. The underwater distributary channel, interdistributary bay, clayey flat, clayey shelf, and overflow microfacies are developed in areas with sufficient sediment supply. The calcareous flat and calcareous shelf are developed in areas with insufficient sediment supply. The deep shelf shale area is divided into deep slope, deep plain, and deep depression microfacies as a result of three breaks. The formation of epicontinental shale with different microfacies is closely related to the tectonic setting, paleoclimate, and sea level rise. The relatively active tectonic setting increases the supply of terrigenous clasts, forming muddy water fine-grained sediment. The warm and humid paleoclimate is conducive to the enrichment of organic matter. The rapid sea level rise is helpful to the widespread black shale.

Geological conditions and exploration potential for shale gas in Upper Permian Wujiaping Formation in the region of western Hubei-eastern Chongqing

The Upper Permian Wujiaping Formation shale in the region of western Hubei and eastern Chongqing has been found qualified for shale gas accumulation in early exploration activities, but the potential for shale gas exploration is still unclear. Based on the core analysis and test data of well Jy3, combined with regional seismic, testing and logging data of this well and wells nearby, the shale distribution, quality, gas-bearing properties, preservation conditions and resource scale of the second member of Wujiaping Formation were discussed. The results showed that the organic-rich shale of deep-water shelf facies and slope to shallow shelf facies were developed and they appeared to be thin in the southwest and thick in the northeast. The shale was mainly composed of quartz (averages 43.10%), clay minerals (average 21.95%) and calcite (averages 17.20%). The organic matter is rich (average TOC value of 8.77%), mainly of type Ⅱ1 kerogen, high thermal maturity (average converted %Ro value of 2.06%), excellent porosity (average value of 5.59%) and good gas bearing property (total gas content averages 3.73 m3/t). The Wujiaping Formation in the Hongxing area is the most promising replacement strata and area for shale gas exploration and development, which shares the largest shale thickness, the best preservation condition, and the most abundant resource.

Biostratigraphy and lithofacies of the Viséan and Serpukhovian deposits of the southeast of the East European platform

This paper presents new data on the biostratigraphy and sedimentology of Upper Visan and Serpukhovian deposits (Lower Carboniferous) from boreholes in the southeast of the East European Platform around the Sol-Iletsk Swell (Orenburg region). Based on rock lithology, sublatitudinal facies zones are recognized in the studied area. The deposits are represented by open shallow shelf facies, up to 500 m thick, in the area of the Sol-Iletsk Swell and near-side zone of the Preuralian Foredeep. To the south, at the margin of the Northern Pericaspian, their thickness sharply decreases to 36 m, and the succession is composed of carbonate-clay sediments of the relatively deep shelf. Based on foraminifers and conodonts, the Upper Visan and Serpukhovian are subdivided using the horizons of the stratigraphic scheme of the East European Platform, and foraminiferal zones are recognized. The base of the Serpukhovian in the high-energy shallow-water facies of the open shelf is drawn at the entry of the foraminifers Janischewskina delicata and Endothyranopsis plana; in lower-energy facies, it is placed at the level of the first appearance of Neoarchaediscus postrugosus; in the clay-carbonate facies of the relatively deep shelf it is based on the appearance of the conodont Lochriea ziegleri. The distribution of microfauna from the five boreholes, the characteristics of the foraminiferal zonal assemblages, and the interregional correlation are discussed.

Upper Viséan and Serpukhovian Biostratigraphy and Lithofacies of the Southeast of the East European Platform

This paper presents new data on the biostratigraphy and sedimentology of upper Visean and Serpukhovian deposits (Lower Carboniferous) from boreholes in the southeast of the East European Platform around the Sol-Iletsk Swell (Orenburg oblast). Using rock lithology, sublatitudinal facies zones are recognized in the studied area. The deposits are represented by open shallow shelf facies, up to 500 m thick, in the area of the Sol-Iletsk Swell and near-side zone of the Preuralian Foredeep. To the south, at the margin of the Northern Pericaspian, their thickness sharply decreases to 36 m, and the succession is composed of carbonate-clay sediments of the relatively deep shelf. On the basis of foraminifers and conodonts, the upper Visean and Serpukhovian are subdivided using the regional substages of the stratigraphic scheme of the East European Platform, and foraminiferal zones are recognized. The base of the Serpukhovian in the high-energy shallow-water facies of the open shelf is drawn by the entry of the foraminifers Janischewskina delicata and Endothyranopsis plana; in lower energy facies, it is placed at the level of the first appearance of Neoarchaediscus postrugosus; in the clay-carbonate facies of the relatively deep shelf, it is drawn according to the appearance of the conodont Lochriea ziegleri. The distribution of microfauna from five boreholes, the characteristics of the foraminiferal zonal assemblages, and the interregional correlation are discussed.

Microstructural investigation of gas shale in Longmaxi Formation, Lower Silurian, NE Sichuan Basin, China

The purpose of this paper is to probe into the nature and variability in vertical microstructure within Longmaxi gas shales of well WX2, Lower Silurian, located in the NE Sichuan Basin of China. To understand the pore system of these rocks, the total porosity, pore size distribution (PSD), surface area, organic geochemistry, and mineralogy, some image analyses by electron microscopy are performed. The obtained results indicated the following. (1) Gas shales have a complex pore structure. Total porosity from helium pycnometry ranges between 0.75% and 4.62%. Total organic content ranges between 3.4 wt.% and 11 wt.%, and vitrinite reflectance measured between 2.5% and 3.1%. The gas shales are quartz and clay rich. (2) Focused ion beam milling and field emission scanning electron microscopy provide high-resolution images of pores distribution and types. Three primary types of pores, organic matter (OM)-hosted pores, mineral-hosted pores, and micro-fractures, and 11 subgroups have been recognized. (3) The N2 isotherm curves are grouped into two types, II b as well as IV a, with the hysteresis loop belonging to Type B, reflecting the morphology of slits pores, and Type A, presenting cylinder pores. Combination of N2 and CO2 isotherm curve shows bimodal in two diameter sections, 0.3–1 nm and 2–5 nm, revealing meso-pores play important role in pore volume, whereas the micro-pores have great influence on pore specific surface area. (4) Total organic carbon (TOC) is considered to be the dominant contributor to total porosity and pore types, with the proportion of OM-hosted pores following the upward decreasing tendency of TOC. In plane, pore types are controlled by sedimentary facies. The OM-hosted pores are more prevalent in deep shelf facies within lower part of shale other than upper formation, instead of shallow shelf facies.

Upper Middle to lower Upper Ordovician chitinozoans and conodonts from the Bliudziai-150 core, southern Lithuania

Integration of chitinozoan and conodont biozones is based on detailed collections from the Bliudziai-150 well, southern Lithuania, that penetrated upper Darriwilian – lower Sandbian sedimentary carbonate rocks. The strata accumulated at the southeastern side of the Livonian Tongue and represent the transition from shallow shelf facies of the Lithuanian shallow-water shelf facies belt to the deeper-water shelf facies belt. Systematically collected and closely spaced samples of the carbonates from the Bliudziai-150 core yielded both chitinozoans and conodonts in abundance. The chitinozoans are assigned to the Laufeldochitina striata, Laufeldochitina stentor, Angochitina curvata – Armoricochitina granulifera, Lagenochitina dalbyensis, Belonechitina hirsuta, and Spinachitina cervicornis chitinozoan biozones, and the conodonts are assigned to the Pygodus serra Zone (with three subzones), Pygodus anserinus Zone (with two subzones), and the three subzones of the Amorphognathus tvaerensis Zone. The fossils are clearly of Baltoscandian affinity, and the integrated results support existing knowledge concerning the biostratigraphy of the two groups in Baltoscandia. Thus, the studied interval is confidently assigned to the Lasnamägi, Uhaku, Kukruse, and Haljala regional stages of the East Baltic regional chronostratigraphical scheme, corresponding to the global upper Darriwilian – lower Sandbian stages (Middle to Upper Ordovician). International correlation of the chitinozoan and conodont succession is presented, and the position of the base of the international Sandbian Stage is placed at 1380.00 m in the Bliudziai-150 core. A horizon marked by multiple discontinuity surfaces represents the base of Kegel depositional sequence and coincides with the appearance of Armoricochitina granulifera, Angochitina curvata, Lagenochitina dalbyensis, and Baltoniodus gerdae in the Bliudziai-150 core.

The natural-gas hydrate exploration prospects of the Nayixiong Formation in the Kaixinling-Wuli Permafrost, Qinghai-Tibet Plateau

Critical components of the Qinghai-Tibet Plateau natural-gas hydrate (NGH) petroleum system has been examined in this study. The results demonstrate that the Kaixinling-Wuli permafrost region contains viable prospects for gas hydrate exploration within favorable temperature and pressure stability conditions. In the study area, the average annual temperature of ground surface is −4.2 °C, the thickness of permafrost ranges from 40 to 150 m (average 84 m), and the geothermal gradient beneath the permafrost is between 1.54 °C/100 m and 2.67 °C/100 m (average 2.03 °C/100 m). The thickness of the methane-gas hydrate stability zone (GHSZ) is approximately 240–450 m. The Upper Permian Nayixiong Formation is dominated by braided delta and shallow shelf facies under mostly reducing conditions. The potential swamp deposited source rocks have a high total organic carbon (TOC) content that features a mixture of kerogen types II or III and an average vitrinite reflectance (Ro) of 2.04%. Overall, the thick sedimentary column in this region, its abundance of organic matter and its high thermal maturity suggest that the Nayixiong Formation source rocks have a high gas-generation potential. An effective fault-fracture-pore system also provides migration channels for deeper gas and also act as a reservoir for vein-type gas hydrate occurrences. The Kaixinling-Wuli area, when compared to other regions in the Qinghai-Tibet Plateau, exhibits greater gas hydrate petroleum system exploration potential as a result of the favorable temperature/pressure stability conditions and effective gas and gas-hydrate migration-storage system.

Relationships between development of organic-rich shallow shelf facies and variation in isotopic composition of pyrite (Middle Triassic, Spitsbergen)

Relationships between development of organic-rich shallow shelf facies and variation in isotopic composition of pyrite (Middle Triassic, Spitsbergen) 51 samples from the Middle Triassic black shales (organic carbon-rich silt-stones; up to 4.9% TOC - Total Organic Carbon) from the stratotype section of the Bravaisberget Formation (western Spitsbergen) were analyzed with respect to isotopic composition of pyritic sulphur (Δ34S) and TOC. Isotopic composition of syngenetic pyrite-bound sulphur shows wide (Δ34S from -26‰ to +8‰ VCDT) and narrow (Δ34S from -4‰ to +17‰ VCDT) variation of the Δ34S in upper and lower part of the section, respectively. Range of the variation is associated with abrupt changes in dominant lithology. Wide Δ34S variation is found in lithological intervals characterized by alternation of black shales and phosphorite-bearing sandstones. The narrow Δ34S variation is associated with the lithological interval dominated by black shales only. Wide and narrow variation of the Δ34S values suggests interplay of various factors in sedimentary environment. These factors include oxygen concentration, clastic sedimentation rate, bottom currents and burrowing activity. Biological productivity and rate of dissimilatory sulphate reduction had important impact on the Δ34S variation as well. Wide variation of the Δ34S values in the studied section resulted from high biological productivity and high rate of dissimilatory sulphate reduction. Variable degree of clastic sedimentation rate and burrowing activity as well as the activity of poorly oxygenated bottom currents could also cause a co-occurrence of isotopically light and heavy pyrite in differentiated diagenetic micro-environments. Occurrence of organic matter depleted in hydrogen could also result in a wide variation of the Δ34S values. Narrow variation of the Δ34S values was due to a decrease of biological productivity and low rate of dissimilatory sulphate reduction. Low organic matter supply, low oxygen concentration and bottom currents and burrowing activity were also responsible for narrow variation of the Δ34S. The narrow range of the Δ34S variation was also due to occurrence of hydrogen-rich organic matter. In the studied section the major change in range of the Δ34S variation from wide to narrow appears to be abrupt and clearly associated with change in lithology. The change of lithology and isotopic values may suggest evolution of the sedimentary environment from high- to low-energy and also facies succession from shallow to deeper shelf. The evolution should be linked with the Late Anisian regional transgressive pulse in the Boreal Ocean.

Devonian ecological-evolutionary subunits in the Appalachian Basin: a revision and a test of persistence and discreteness

Abstract New data compilations for successive formation scale intervals, approximately third-order sequences, permit a statistical characterization of the ecological–evolutionary subunits (EESUs) or faunas of the Latest Silurian to mid-Late Devonian interval in the Appalachian Basin. Cluster analysis using the Jaccard coefficient of similarity show that certain formations cluster tightly together on the basis of faunal composition while in other cases units are sharply set off from superseding units. This result indicates both the coherence of faunal composition within EESUs and the discreteness of their boundaries. The results also require minor revision of EESUs previously delineated, including the addition of three new units. The Esopus Formation is designated as a distinct unit separate from the Schoharie on the basis of brachiopods from shallow water facies of the Skunnemunk outlier in New York; in addition, a short-lived Stony Hollow fauna is recognized in shallow shelf facies of the Union Springs Formation and coeval units (formerly referred to as the lower Marcellus Formation) and the lower transgressive beds of the overlying Oatka Creek Formation. This fauna, consisting of subtropical Old World Realm (OWR) emigrants, is distinctive from both the underlying Onondaga fauna and that of the overlying Hamilton Group. Moreover, the Tully Formation presents another case of a short-lived incursion of tropical OWR taxa followed by the extermination of this fauna and reappearance of a suite of typically Hamilton taxa. This case illustrates that EESUs may persist globally despite their local extermination or emigration from a large region, such as the Appalachian Basin. Review of the broader context of EESU turnover suggests that these crises are geologically rapid and synchronous. Moreover, most of the Devonian EESU boundaries coincide with recognized global bioevents, within the limits of combined biostratigraphic and sequence stratigraphic resolution. Hence, these crises, although perhaps locally accentuated in the Appalachian Basin, are allied to global causes. They appear mostly to be associated with rapid rises in sea level, periods of widespread climatic change and hypoxic events in basinal areas.

Phylogeny of the Reedocalymeninae (Trilobita): implications for Early Ordovician biogeography of Gondwana

Abstract The shallow-water Neseuretus Association has been recognized by many authors as an important indicator of the former extent of Gondwana during the Early Ordovician. Phylogenetic relationships of the Reedocalymeninae (Arenig-?Early Silurian) were investigated using cladistic analysis, incorporating 22 species of Neseuretus as well as representatives of all other reedocalymenine genera. The area cladogram derived from this analysis contains four subclades each containing areas that span much of the palaeogeographic extent of Gondwana, which are interpreted as representing separate biogeographic events during the evolution of the subfamily. As the majority of taxa included in the analysis are associated with shallow shelf facies, this suggests that no significant environmental barriers existed across the continent during the Early Ordovician. Consideration of area relationships both for different subclades within the area cladogram, and within a consensus area cladogram, also supports the idea of a faunal cline between eastern and western Gondwana during this time interval. Further cladistic analysis of different trilobite taxa can be used to test these ideas.

Distribution and control of mixed terrigenous‐carbonate surficial sediment facies, Wanganui shelf, New Zealand

Abstract Shallow‐marine, mixed temperate, terrigenous‐carbonate sediments are accumulating on the shallow to deep Wanganui shelf (c. 20–110 m water depth). From cluster analysis of textural and compositional parameters, five surficial sediment facies and five subfacies have been defined for the shelf. Facies 1 (Bivalve‐bearing [gravelly] sand), is a terrigenous‐dominated, shallow shelf facies ( 50% CaCO3, and occurs in the shallow to mid shelf region (c. 30–90 m) where levels of terrigenous input are reduced, and substrates and hydraulic conditions are favourable for carbonate production. Facies 2 comprises Subfacies 2a (Bivalve‐dominated sandy gravel), 2b (Bryozoan/bivalve‐dominated sandy gravel), and 2c (Bryozoan/bivalve‐dominated muddy sandy gravel and ...

Acadian dextral transpression and synorogenic sedimentary successions in the Appalachians

The successive Seboomook-Littleton (northern Appalachians) and Catskill-Pocono (central Appalachians) clastic wedges suggest oblique convergence and southwestward migration of Acadian orogeny beginning in Early Devonian and continuing into Early Mississippian. Wrench-fault movement in Maritime Canada coincided with deposition of all but the earliest components of the Catskill-Pocono clastic wedge and continued into the Pennsylvanian. Contrasts between a thin, Lower to Middle Devonian shallow-shelf facies in the Alabama Appalachian fold-thrust belt and a time-equivalent, thick, shallowing-upward sedimentary to volcanic succession in the adjacent Talladega slate belt are interpreted to reflect a wrench-fault basin. A wrench-fault setting for Devonian rocks in Alabama integrated with manifestations of oblique convergence during the Acadian orogeny in the central and northern Appalachians can be accommodated in dextral transpression along the entire length of the Acadian Appalachian orogen.

Petroleum Reservoirs in Silurian Dolomite of Western Illinois: ABSTRACT

The basal Silurian Alexandrian carbonates, sandwiched between the Ordovician-Silurian and Silurian-Devonian unconformities, have produced more than 3 million bbl of oil from the Buckhorn East and Brooklyn fields on the western margin of Illinois. These fields are shallow (450-670 ft), remote from the most-productive portion of the Illinois basin, and potential exists for additional production from this unconformity play. Three lithofacies were recognized in Alexandrian carbonates in detailed core and wireline studies that included thin-section, x-ray, and SEM analyses. These carbonates are regressive sequences composed of shallow-shelf, restricted-subtidal, and intertidal facies. The shallow-shelf facies is primarily biosparite composed of crinoid and brachiopod debris; t is facies is dolomitic in places. The restricted-subtidal facies consists of both bioturbated and undisturbed gray micrites containing stylolites, green chloritic clay, and glauconite; sparry clacite fills vertical cracks. The intertidal facies is tan, mottled, vuggy dolomite composed of algal mats, anhydrite-filled birdseye structures, burrows, and scattered marine fossils. Petroleum reservoirs apparently developed as a result of dolomitization in intertidal and shallow-shelf carbonates. Vuggy porosity, a possible result of karst leaching, is found in pay zones within the dolomitic intertidal facies. Evidence of emergence and subaerial weathering include the presence of vadose silt, anhydrite, and nontectonic breccia. End_of_Article - Last_Page 306------------

Depositional Patterns of Kerogen, Atlantic Margin, North America: ABSTRACT

Geochemical and biostratigraphic data from offshore wells along the Atlantic margin of North America define a depositional history dominated by coastal-plain and shallow-shelf facies containing degraded and residual continent-derived kerogen. Exceptions to this generalization are 4 depositional facies containing hydrogen-rich amorphous kerogen assemblages. The rocks containing hydrogen-rich amorphous kerogen assemblages are: (1) Upper Jurassic inner-shelf facies probably deposited in algal-rich lagoonlike environments, (2) Lower Cretaceous nonmarine coaly facies, probably deposited in algal-rich swamplike environments, (3) middle Cretaceous abyssal-plain facies probably deposited by turbidity currents that originated on an algal-rich slope, and (4) Miocene outer-shelf to upper-slope facies probably deposited under algal-rich upwelling systems. Correlation of these facies to seismic packages allows for extrapolation of probable organic facies distribution throughout the continental margin. Such modeling of organic facies distributions in conjunction with plate-tectonic and ocean-circulation models permits refinement of strategies for hydrocarbon exploration. End_of_Article - Last_Page 234------------

Depositional Environments of Marine Pleistocene Deposits in Southeastern North Carolina

Outcropping marine, fossiliferous Pleistocene beds in southeastern North Carolina (Neuse Formation) consist of four facies, each facies having distinct lithologic and faunal characteristics: (1) bay facies—clay to fine-grained quartz sand, Retusa-Nuculana molluscan assemblage, and Elphidium-Ammonia foraminiferal assemblage; (2) shallow shelf facies—fine-grained quartz sand, Donax-Spisula molluscan assemblage, and Elphidium-Ammonia-Hanzawaia foraminiferal assemblage; (3) lagoonal facies—very fine-grained quartz sand and sand-silt-clay, Crassostrea-Gemma molluscan assemblage, and Elphidium-Ammonia foraminiferal assemblage; (4) high-energy facies—coquina, Donax-Mercenaria molluscan assemblage, and Quinqueloculina foraminiferal assemblage. The bay facies seems to have been deposited in a body of water in which polyhaline and marine salinities prevailed and which was moderately exposed to the open ocean. The shallow shelf facies probably formed in the sublittoral zone in the open ocean. The lagoonal facies seems to have formed in a body of water where mesohaline and polyhaline salinities were dominant. The high-energy facies was probably deposited in inlets between barrier bars or in shoal areas near the littoral zone. The water temperature during deposition of the Neuse Formation seems to have been slightly higher than that now found in North Carolina coastal waters. Changes in temperature and shoreline configuration after Neuse deposition seem to have had a marked effect on molluscan latitudinal migration.

Stratigraphy and Depositional History of the Gaptank Formation (Pennsylvanian), West Texas

The base of the Gaptank Formation is redefined as the base of the conglomerate member, and the underlying Chaetetes -bearing limestone member is transferred to the Haymond Formation. This change gives the Gaptank Formation a more distinctive lithologic base having greater areal continuity. The Gaptank Formation is subdivided into a conglomerate member (bottom) and a limestone member (top) that intertongue, in part, with a sandstone and shale member (middle). Gaptank strata display cyclical repetition and illustrate differences in deposition arising from differences in subaqueous topography. Three submarine land forms persisted as distinctive depositional sites: a shallow shelf (undaform), a sloping surface (clinoform), and a basin (fondoform). Progressive basinward extension of the shallow-shelf areas resulted in more widespread shallow-water environments during deposition of the Gaptank Formation. In outcrop, many individual Gaptank limestone beds can be traced laterally from a shallow-shelf facies into a shelf-edge facies, then into an upper-slope facies. Other modifications in the sedimentary pattern resulted from gentle warping in different areas and at different times during deposition. Comparison of lithologic and depositional features in the Gaptank Formation with those of the late Pennsylvanian strata along the eastern shelf of the Midland basin in central and north-central Texas shows striking similarities.