Detrital zircon U–Pb geochronology and geochemistry of the Riachuelos and Palma Sola beach sediments, Veracruz State, Gulf of Mexico: a new insight on palaeoenvironment

Zircons are abundant in the beach sediments. In this study, surface microtexture, mineralogy, bulk sediment geochemistry, trace element composition and U–Pb isotopic geochronology of detrital zircons collected from the Riachuelos and Palma Sola beach areas, southwestern Gulf of Mexico were performed to infer the sediment provenance and palaeoenvironment. The zircon microtexture was categorized as mechanically- and/or chemically-induced features. The weathering index values for the Riachuelos (~ 72–77) and Palma Sola (~ 71–74) beach sediments indicated moderate weathering of both of the two source areas. The major and trace element data of bulk sediments suggested passive margin settings for the two areas. The trace elemental ratios and chondrite-normalized rare earth element (REE) patterns of bulk sediments revealed that the sediments were likely sourced by felsic and intermediate igneous rocks. And the zircon Th/U ratios (mostly more than 0.2) and zircon REE patterns (with negative Eu and positive Ce anomalies) suggested a magmatic origin for both of the beach sediments from these two areas. Two distinct zircon age peaks respectively belonging to the Paleozoic and the Cenozoic were identified both in the Riachuelos and Palma Sola beach sediments. Zircon geochronology comparison research between the Riachuelos–Palma Sola beach sediments and potential source areas in SW Gulf of Mexico revealed that the source terrane supplied the Paleozoic zircons of this study was identified as the Mesa Central Province (MCP), and the Cenozoic zircons were transported from the nearby Eastern Alkaline Province (EAP). Moreover, although the Precambrian zircons were very few in the studied sediments, their geochronology and geochemistry results still could infer that they were contributed by the source terranes of Grenvillian igneous suites in the Oaxaca and the Chiapas Massif Complexes.


Introduction
Zircon is a common accessory mineral in clastic sediments, which retains U-Pb isotopic signatures during erosion, recycling, and transport; differences in zircon age populations in sediments are linked to the nature and age of source terranes (Ji et al. 2019;Joy et al. 2019;Liu et al. 2019;Chaudhuri et al. 2020). Hence, detrital zircon U-Pb age geochronology is reliable to infer provenance, as well as helpful to discriminate sediments derived from various source terranes and to understand their sediment-transport pathways Madhavaraju et al. 2018;Wang et al. 2018a). Although U-Pb ages of detrital zircons have been used widely in different studies to investigate sediment provenance, few studies recommended that the combination of zircon age data and trace element data is more reliable to better understand the depositional history of a complex depositional basin (Wang et al. 2017;Li et al. 2019).
On the other hand, the importance of mineralogical and geochemical composition of clastic sediments to infer intensity of weathering, provenance, and tectonic setting of the source region is well established in the literature (Basu et al. 2016;Carranza-Edwards et al. 2019;Casse et al. 2019;Karudu 2019;Men et al. 2019;Kettanah et al. 2020). Although clastic sediments are affected by factors such as weathering, diagenesis, and mineral fractionation during transport, their mineralogical-geochemical composition is mainly related to the parent rock types and the influence factors of the source (McLennan et al. 1993;Basu 2017;Ramos-Vázquez et al. 2017Bansal et al. 2018;Ndjigui et al. 2019;Rivera-Gómez et al. 2020). Therefore, the geochemical composition of clastic sediments is also reliable to infer the tectonic setting of a sedimentary basin. The immobile trace elements and their elemental ratios differ widely between felsic and mafic rocks, hence are particularly helpful to discriminate the sediments derived from felsic (high in Zr, Nb, Hf, Y, and Th) or mafic (high in Cr, V, Ni, and Sc) sources (Cullers and Podkovyrov 2002;Tawfik et al. 2017). Similarly, different source rocks can be identified by the value of Eu anomaly, for instance, sediments derived from mafic igneous rocks, especially basalt, exhibit positive Eu anomalies or no Eu anomaly (Basu 2017;Wang et al. 2018b;L wen et al. 2020). In addition, the mobile elements like K, Na, and Ca among major elements are useful to infer the weathering intensity and other palaeoenvironmental conditions of the source area (Barros dos Santos et al. 2019).
Textural characteristics and geochemical compositions of beach sediments along the Gulf of Mexico have been studied by some authors (Rosales-Hoz et al. 2008;Tapia-Fernandez et al. 2017;Hernández-Hinojosa et al. 2018;Ramos-Vázquez et al. 2018). And many studies have been focussed on the zircon U-Pb ages of volcanic rocks from various terranes in Mexico, like Chiapas, Oaxaca, Zacatecas, and Xolapa, with little consideration to the detrital zircons in beach sediments (Keppie et al. 2003;Weber et al. 2009Weber et al. , 2018Talavera-Mendoza et al. 2013;Escalona-Alcázar et al. 2016;Cisneros de León et al. 2017;Peña-Alonso et al. 2017Ramírez-Peña and Chávez-Cabello 2017). Recently, based on the geochemical composition of sandstone and the detrital zircon U-Pb geochronology, Wengler et al. (2019) interpreted the provenance of a sedimentary unit in the Mesa Central region, central Mexico. However, investigations based on the comprehensive research of mineralogy, detrital zircon U-Pb geochronology, major and trace element geochemistry of bulk sediment in the Gulf of Mexico beach area to infer the sediment provenance, are meagre.
The degree of grain roundness and microtextures on quartz grain surfaces are considered as powerful tools to infer sediment provenance and have been applied in various studies to reconstruct palaeoenvironments (Margolis and Krinsley 1974;Mahaney 2002;Madhavaraju et al. 2009;Mahaney et al. 2012;Vos et al. 2014;Kalińška-Nartiša et al. 2018;Chmielowska and Woronko 2019). However, microtextures, especially on zircon grains, are not studied sufficiently, possibly because of the difficulties in separating zircon grains from rocks and sediments. To fulfil this gap, the microtextures of detrital zircons from the Riachuelos and Palma Sola beach sediments are surveyed, and the provenance information based on the types of surface features is briefly discussed.
Furthermore, in this study, the mineralogy and geochemistry of bulk sediments, U-Pb and trace element data of detrital zircons collected from the Riachuelos and Palma Sola beach areas are analysed, to investigate the source areas supplying sediments to the beach areas and to infer the palaeoenvironments.
The geology of coastal regions of the Gulf of Mexico is shown in Fig. 1. The outcrops along the Gulf of Mexico coast are mainly composed of the Precambrian-Paleozoic metamorphic rocks comprising schist and gneiss, the Mesozoic-Cenozoic clastic and calcareous sedimentary rocks, the Cenozoic mafic and intermediate volcanic rocks, and the Quaternary volcanic rocks and clastic sediments (Keppie et al. 2003;Kasper-Zubillaga et al. 2019).
The Gulf of Mexico littoral zone belongs to the wavedominated transgressive marginal sea coast with narrow coastal plain (Inman and Nordstrom 1971;Davis 1988;Boyd et al. 1992;Carranza-Edwards 2011). The Riachuelos coast is wider than the Palma Sola coast. The easternmost side of the Trans-Mexican Volcanic Belt (TMVB) ends at the Veracruz State area, which formed a cliff at northern Palma Sola beach , and conduced longshore surface currents of northward flow direction in summer with an average velocity of about 4.5 cm/s. Higher wind velocities were recorded during summer, varying from 3.0 m/s to 5.4 m/s (Yañez-Arancibia and Day Jr. 1982). However, during winter, longshore surface currents flow towards south with an average velocity of 6 cm/s. Monreal-Gómez and Salas de León (1990) documented that Gulf of Mexico water circulation and hydrodynamic conditions are controlled by loop currents and anticyclonic rings. The climate for Gulf of Mexico is considered as sub-humid to humid conditions (Tamayo 1991).
The major rivers feeding sediment to the western and southern areas of the Gulf of Mexico are Pánuco, Tecolutla, Jamapa, Papaloapan, Coatzacoalcos, Grijalva and Usumacinta (Fig. 1). River Pánuco is 510 km long with a drainage area of~98,227 km 2 , which drains into the Gulf of Mexico at Tampico State (Fig. 1). The drainage area of River Pánuco covers parts of Trans-Mexican Volcanic Belt (TMVB) and the Sierra Madre Oriental (SMOr) and its sediment lithology consists of Upper Jurassic-Lower Cretaceous carbonate rocks, Upper Cretaceous marine flysch-type clastic sediments and Pliocene-Holocene marine clastic sediments. River Tecolutla is one of the major rivers in Mexico, starts from the Xolapa uplift that is the easternmost extension of Mexican Volcanic Belt (MVB; Figs. 1 and 2), and, flows through and covers a large volcanic terrane (Self 1975) of an Neogene to Quaternary geological province extending through Central Mexico for about 1000 km (Verma 2009(Verma , 2015. River Jamapa flows for about 368 km from the Pico de Orizaba (19°02′N and 97°16′W; volcano Citlalépetl) eastward emptying into the Gulf of Mexico at Boca del Rio, Veracruz State. The sediment lithology of River Jamapa draining area is dominated by the Upper Cretaceous marine flysch, and the Pliocene-Holocene volcanics and marine clastic sediments (Fig.  1). River Papaloapan joins in the Gulf of Mexico at Alvarado, Veracruz State, with an annual water discharge of 39,175,000 m 3 (Tamayo 1991). The drainage area of River Papaloapan is the second largest hydrological basin (17°-19°N and 95°-97°W) in Mexico, which partly covers the Cuicateco terrane (39,189 km 2 ; Fig. 2). River Coatzacoalcos originates in the Oaxaca State area with a mean annual discharge of 32,732 hm 3 (Tamayo 1991), and generates a drainage basin of between 17°46′-18°10′N and 92°25′-94°31′W. The sediment lithology of the River Coatzacoalcos draining area consists mainly of intrusive igneous rocks and metasedimentary rocks of the Oaxacan Complex. River Grijalva flows for about 640 km from the Chiapas State area of southeastern Mexico. River Usumacinta, originated from the northwestern region of neighbouring Guatemala, is considered as the longest river of Mexico with~1100 km length, and ranks second among the freshwater rivers flowing into the Gulf of Mexico (Muñoz-Salinas et al. 2017). The Grijalva and Usumacinta Rivers join upstream in about 50 km before they enter the Gulf of Mexico together (Muñoz-Salinas et al. 2017).

Material and methods
In total, 35 bulk sediment samples (~2 kg each) were collected from the Riachuelos (number of samples n = 15) and Palma Sola (n = 20) beach areas, where the waves reach the coast during high tide.
The mineralogy of 10 sediment samples in different size fractions (fine-and medium-grained) was identified using the Shimadzu XRD-6000 diffractometer at the Institute of Geology, Universidad Nacional Autónoma de México (UNAM). X-Ray diffraction (XRD) was operated with an accelerating voltage of 40 kV and a filament current of 30 mA, using CuKα radiation and a graphite monochromator.
Scanning Electron Microscopy (SEM) was used to infer the morphological features and shapes of the zircon grains. Fifteen zircon grains were randomly handpicked from each sediment sample under a stereomicroscope, and then 150 zircon grains were selected for microtexture analysis. Grains coated with gold and palladium were mounted on 3 mm-thick carbon discs and examined by JEOL-JSM-6360LV SEM with general magnifications between × 180 and × 5500 at Instituto de Ciencias del Mar y Limnología, UNAM. The quantitative compositional data of randomly-selected zircon grains (7 grains in each sample) were determined by JEOL-JXA-8900R SEM equipped with Energy Dispersive X-Ray Spectrometry (SEM-EDS) at UNAM.
Major element concentration of total 35 bulk sediment samples collected from the Riachuelos and Palma Sola beach areas were analysed by Rigaku RIX-3000 X-Ray Fluorescence at the Institute of Geology, UNAM. Accuracy of major element analysis was monitored by the standard JGB1 (GSJ). The precision of major element data was better than 5%. Loss on ignition was obtained by weighing after 1 h combustion at 1000°C. Trace element concentration of 30 bulk sediment samples were determined by a VG Elemental PQII plus ICP-MS and the operation procedure was similar as details in Jarvis (1988). The United States Geological Survey Standard BCR-2 (Basalt, Columbia River) was used for trace element data calibration, and the analytical precision was less than 5% in general.
Two bulk sediment samples, one from Riachuelos beach area (RIA4) and another from Palma Sola beach area (PAL20) were selected for detrital zircon U-Pb dating. In total, 263 detrital zircon grains (146 grains from the Riachuelos and 117 grains from the Palma Sola) were handpicked from bulk sediments, mounted in epoxy resins and polished. Cathodoluminescence images were taken using an ELM 3R luminoscope to reveal zircon internal texture. All 263 zircon grains were analysed for obtaining information of trace element (Nb, Hf, Th, U, Pb) and REE geochemistry, and U-Pb isotope geochronology.
Zircon U-Pb dating and trace element concentration analyses were simultaneously conducted using a LA-ICP-MS coupled with Thermo Xii Series quadrupole mass spectrometry, followed by the methodology described by Solari et al. (2018). ANIST 610 glass standard was used to recalculate the trace element concentration, by normalizing them with 29 Si. U and Th concentrations  Paton et al. (2010). 207 Pb/ 206 Pb ratios, ages and errors are calculated according to Petrus and Kamber (2012). Individual analyses with > 10% uncertainty, > 20% discordance, or > 5% reverse discordance were not considered for interpretation. The U-Pb concordia-age plots, the probability density distribution histograms were plotted using ISOPLOT software ; e Euhedral zircon with one side broken edge (bez), probably due to collision (bec); f Zircon grain with bulbous edge (ble) and reworked conchoidal fracture (rcf); g Grain showing collision impact point (clp), collision fracture (cf) with straight groove (sgr); h Grain with broken edge due to collision (bec); i Enlarged view of broken edge of grain h showing arc-shaped steps (as); j Grain with cavities (gc), adhered particles (ad), solution and precipitation features (s/p); k K-feldspar (kf) adhering to a zircon grain, also showing collision fracture (cf), and solution and precipitation features (s/p); l Grain with silica pellicle (sp), adhered particle (ad), and solution and precipitation features (s/p) (Ludwig 2008). Further detailed analytical methods followed to measure zircon U-Pb isotopes by LA-ICP-MS were the same as descriptions in Solari et al. (2018). Comparison research between the detrital zircon U-Pb dating ages tested in this study with the previously published age data (Cantagrel and Robin 1979;López-Infanzón 1991;Nelson and González-Caver 1992;Ferrari et al. 2005;Weber et al. 2009Weber et al. , 2018Solari et al. 2011;Ortega-Obregón et al. 2014 Dual striated zircon (dsz) well abraded at both ends; b Zircon grain with an abraded edge (abe) and adhered particles (ad); c Zircon grain with an abraded edge (abe) and silica pellicle (sp); d Angular grain with circular solution pits (csp) on fracture plane and silica pellicle (sp); e Collision fracture (cf) with adhered particles (ad); f A sub-rounded zircon grain with adhered particles (ad); g A euhedral zircon grain with collision fracture (cf); h Zircon grain showing delamination (dl), meandering ridge (mr), and adhered particle appears to be silica globule (ads); i Enlarged view of grain h showing delamination (dl); j V-shaped percussion cracks (vs) and arc-shaped steps (as); k Reworked conchoidal fracture (rcf) with linear steps (ls); l Zircon grain with adhered particles (ad), silica globule (sg), merged silica globule (msg), and silica flower (sf), due to precipitation   Fig. 2) was conducted to investigate the provenance of the Riachuelos and Palma Sola beach sediments.

Mineralogy of bulk sediments
X-Ray diffraction analysis reveals that zircon, quartz, magnetite, and ilmenite are the major and chromite, geikielite, spinel, and labradorite are the minor minerals in the Riachuelos (Fig. 3a, b) and Palma Sola (Fig. 3c, d) beach sediments. Based on the quantitative compositional data (Additional File 1: Supplementary Information 1) obtained by SEM-EDS analysis, heavy minerals such as apatite

Zircon CL images
Zircons collected from the Riachuelos and Palma Sola beach sediments are mostly prismatic and showing oscillatory zoning with luminescent overgrowths, which supports for a magmatic origin (Fig. 5). More cathodoluminescence images for the zircon sequences of the Riachuelos (Supplementary Information 8) and Palma Sola (Supplementary Information 9) beach sediments are provided in Additional File 2 to better recognize their grain morphology. In general, the morphology differences between zircons from the Riachuelos and the Palma Sola beach sediments are not significant (Fig. 5).

Zircon surface microtextures
To infer the sedimentary provenance, transportation medium and depositional history of the beach sediments, microtextures on detrital zircon grains from the Riachuelos and Palma Sola beach sediments are investigated and interpreted based on the traditional classification procedures followed by various researchers ( Table 1, and their SEM images are shown in Figs. 6 and 7, respectively. The mechanical features identified in the Riachuelos zircons include well-abraded euhedral grains with broken edges (Fig. 6a, b, e, h, i), crescentic gouge (Fig. 6c, d), arc-shaped steps and linear steps (Fig. 6d, i), bulbous edge and reworked conchoidal fracture (Fig. 6f), and, collision fractures with straight groove (Fig. 6g, h, k). Similarly, the microtextures of chemical origin identified in the Riachuelos zircons are cavities, adhered particles (ad), solution and precipitation features (s/p), and silica pellicle (sp) (Fig. 6j-l).

Geochemical composition of detrital zircons
The trace (Nb, Hf, Th, and U) and REE concentrations of detrital zircons from the Riachuelos and Palma Sola beach sediments are listed in Additional File 1 (Supplementary Information 2 and 3, respectively). The variation in Y content between the Riachuelos zircons (~221-9300 ppm, n = 146) and the Palma Sola zircons (~266-9400 ppm, n = 117) is not significant. However, the variations in Th and U contents of the Palma Sola zircons (~21-2900 ppm and~28-2290 ppm, respectively) are slightly higher than those of the Riachuelos zircons (~16-2017 ppm and~31-1451 ppm). The Th/U ratios of detrital zircons are generally used to differentiate igneous and/or metamorphic origins: Th/U ratio for zircons of igneous origin is > 0.3 and for zircons of metamorphic origin is < 0.1 (Rubatto 2002). The Th/U ratios of the Riachuelos and Palma Sola detrital zircons, respectively varying between 0.14 and 1.56 (0.67 ± 0.29, n = 146) and varying between 0.13 and 2.27 (0.68 ± 0.36, n = 117), are both consistent with a magmatic source (Fig. 9).

Principal component analysis (PCA)
The PCA for the Riachuelos beach sediments demonstrates two major factors ( Fig. 13; Additional File 1: Supplementary Information 6). Factor F1 reveals significant negative loading, which represents the association of a few major (TiO 2 , Fe 2 O 3 , MnO, and MgO), trace (V, Y, and Zn), and rare earth elements (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) related to heavy minerals like magnetite and ilmenite. Factor F2 shows positive loading with elements, which is largely associated with detrital materials. On the other hand, PCA for the Palma Sola beach sediments reveals three factors (Additional File 1: Supplementary Information 7). Factor F1 represents high positive loading with the association of alkali (K and Rb), alkaline (Ba, Mg, Nd, and Ca), and rare earth elements (Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Pb, and Lu), related to the detrital origin. Factor F2 shows a negative loading for elements of Be, Cu, Ga, Li, Sn, Ta, Th, and Zn. Factor F3 represents a negative loading with variables of TiO 2 , Fe 2 O 3 , Co, Cr, Hf, V, and Zr, which are associated with heavy minerals like ilmenite, magnetite, chromite, and zircon. Therefore, the combined results obtained from the Pearson's Correlation Coefficient and the Principal Component Analysis indicate the association of major and trace elements with detrital materials rather than biogenic components.  Harnois 1988) have been widely used to infer intensity of weathering (e.g., Ota et al. 2017;Anaya-Gregorio et al. 2018;Raza and Mondal 2018;Tawfik et al. 2018;Hossain et al. 2018;Kettanah et al. 2020). However, a few authors (Cullers 2000;Buggle et al. 2011;Garzanti et al. 2015) documented that sediments are affected by various factors like sorting, recycling, and having significant proportions of carbonate or phosphate minerals. Hence, care should be taken when applying CIA and CIW indices to infer the intensity of weathering, because CaO content in sediments decrease the values and may mislead interpretation. These authors further recommended to eliminate the CaO content from the CIA and CIW indices and suggested a simply modified index called CIX (CIX = 100 × [Al 2 O 3 /(Al 2 O 3 + Na 2 O + K 2 O)]), which eliminated CaO content (Cullers 2000;Buggle et al. 2011;Garzanti et al. 2014). In general, similar to other indices, the CIX values increase if weathering increases and in highly weathered sediments the values are always more than 80. The calculated CIX values for the Riachuelos and Palma Sola beach sediments vary from 74 to 77 and from 71 to 74, respectively (Table 2), suggesting a moderate intensity of chemical weathering for the source area.
In clastic sediments, Th/U, Rb/Sr, and Ba/Sr ratios can be used as indicators to infer intensity of weathering, because Th, U, Rb, Ba, and Sr are sensitive to chemical weathering and their values are always high in extremely weathered sediments (McLennan et al. 1993;Xu et al. 2010). Similarly, Th/U ratios in sediments increase with increasing weathering due to oxidation and loss of uranium. Th/U ratios above four are expected to be indicative of weathering (McLennan et al. 1993). The average Rb/Sr and Ba/Sr ratios in the Riachuelos (0.09 ± 0.01 and 1.17 ± 0.08, respectively) and Palma Sola (0.18 ± 0.02 and 1.79 ± 0.18, respectively) beach sediments are lower than in UCC (0.32 and 1.57, respectively; Taylor and McLennan 1985). Similarly, average Th/U ratio in the Riachuelos and Palma Sola beach sediments are both less than 4 (3.0 ± 0.7 and 3.3 ± 0.3, respectively; Table 3). Hence, the Rb/Sr, Ba/Sr, and Th/U ratios in the Riachuelos and Palma Sola beach sediments suggest that the intensity of weathering is moderate in the source area.

Source rock composition and tectonic setting
Trace element concentrations (e.g., Th, U, Y, Sc, Cr, Co, Ni, V, and REE) in clastic sediments are more reliable indicators of provenance than major elements and are extensively applied in various studies to infer sediment provenance (e.g., Basu 2017;Zhang et al. 2018;Casse et al. 2019). This is because incompatible trace elements are concentrated in sediments derived from felsic igneous rocks, whereas compatible elements are enriched in sediments derived from mafic igneous rocks and their weathered products. Then, elemental ratios such as La/ Sc, Th/Co, and Th/Sc are significantly different between felsic and mafic rocks and can be used to infer sediment provenance (Cullers and Podkovyrov 2002). Bivariate (Fig. 14a, b) and ternary diagrams (Fig. 14c, d) show that the Riachuelos and Palma Sola beach sediments are plotted between dacite and andesite average compositions, suggesting the provenance of felsic and intermediate igneous rocks. Less contributions from mafic rocks are also revealed by the lower concentrations of Co, Ni, and V in the Riachuelos and Palma Sola beach sediments relative to in the UCC (Fig. 8b).
The Riachuelos and Palma Sola beach sediments are little fractionated (LREE/HREE =~7. 16-9.54 and~6.23-8.56, respectively) and are characterized by both negative and  Table 4), suggesting that the sediments were derived from both felsic and intermediate igneous rocks. Furthermore, the chondrite-normalized REE patterns of the Riachuelos and Palma Sola beach sediments are compared with potential source rocks such as rhyolite, dacite, and basaltic andesite located in the Gulf of Mexico coastal areas (Fig. 15). The similarities in REE patterns between the two beach sediments and source rocks are consistent with the interpretation that the detritus was supplied by felsic and intermediate rocks.
The major and trace element concentrations of detrital sediments have been widely used in various studies to infer the tectonic setting of a sedimentary basin, because detrital sediment composition varies significantly between active and passive margin settings (Bhatia 1983;Roser and Korsch 1986;Saha et al. 2010Saha et al. , 2018Verma and Armstrong-Altrin 2013;Armstrong-Altrin 2015). Although many studies continuously use the old tectonic discrimination diagrams of Bhatia (1983) and Roser and Korsch (1986), in this study, the recently proposed discriminant function diagrams of Verma and Armstrong-Altrin (2016) are considered (Fig. 16). These new statistically discriminant diagrams are efficient to discriminate siliciclastic sediments of island or continental arc, continental rift, and/or collision settings. On these diagrams, the Riachuelos (Fig. 16a, c) and Palma Sola (Fig. 16b, d) beach sediments are divided well within the rift and passive margin fields, suggesting a passive margin setting for the beach areas, which is consistent with the tectonic setting of the Gulf of Mexico Centeno-García 2017).  Taylor and McLennan (1985)

Probable provenance based on zircon surface microtextures
The microtextures identified on zircon grain surfaces (Figs. 6, 7) and their implications on provenance are briefly discussed.
The collision fractures, V-shaped percussion cracks, meandering ridges, arc and linear steps indicate highenergy grain to grain impact and mechanical grinding by littoral transport, as well as high-energy subaqueous deposition such as in river and coastal environments (Finzel 2017;Hossain et al. 2020). However, the mechanical features on the grain surface may likely be associated with the hydrodynamic conditions during littoral transport and wave action in the nearshore zone (Kalińška-Nartiša et al. 2018). The abraded grains with bulbous edges identified in a few zircons show the evidence of high-energy collision through saltation in an aeolian environment (Mahaney 2002;Chmielowska and Woronko 2019). Angular grains with broken edges indicate a short transport in littoral environment as well as a highenergy subaqueous environment, where the breaking of waves is probably the transport agent (Costa et al. 2017). Conchoidal fracture is one of the most dominant microtextures on zircon surfaces identified in both beach areas, which indicate liberation of grains from crystalline rocks (Madhavaraju et al. 2009). In addition, collisions between zircon grains and pebbles may also cause fracturing, as well as delamination (Vos et al. 2014).
The etching process is revealed by chemical features like solution pits and precipitation features associated with the percolation of seawater. The silica globules, silica pellicles, and silica flowers indicate that zircon grains were subjected to precipitation with silica saturated solutions for a considerable time. Vos et al. (2014) documented that when precipitation on grain surface continues, silica globules start to merge to form silica flowers and pellicles.
Overall, the mechanical and chemical features identified on the zircon surfaces of the Riachuelos and Palma Sola beach sediments suggest their transport processes by both littoral and aeolian currents, and a high-energy subaqueous coastal depositional environment.

Probable provenance based on detrital zircon U-Pb ages
U-Pb dating ages of the detrital zircons of the Riachuelos and Palma Sola beach sediments are compared with other zircon U-Pb ages reported by various authors from different terranes of Gulf of Mexico (Fig. 17), and are briefly discussed, to infer the provenance.

Relative zircon age probability distribution -Proterozoic
The Riachuelos beach sediments own three Proterozoic zircons with U-Pb dating ages varying from 1370 Ma to 903.8 Ma and the Palma Sola beach sediments own nine Proterozoic zircons with U-Pb ages varying from 1652 Ma to 629.9 Ma. The possible source terranes contributed to Proterozoic zircons of the Riachuelos and Palma Sola beach areas are the Chiapas Massif Complex and the Oaxaca Complex, where the documented zircon U-Pb ages vary from~1500 Ma to~450 Ma (Keppie et al. 2003;Weber et al. 2012). In addition, Escalona-Alcázar et al. (2016) and Wengler et al. (2019) also reported Proterozoic zircons in the Mesa Central Province, and they inferred that Oaxaca Complex and Chiapas Massif Complex are potential source terranes for the Mesa Central Province.

Relative zircon age probability distribution -Paleozoic
Detrital zircons of the Riachuelos and Palma Sola beach areas also show the Paleozoic ages (~480-252.5 Ma). However, Riachuelos beach sediments are more dominated by the Paleozoic zircons (n = 30) than Palma Sola beach sediments (n = 15). The most likely potential source terrane for the Paleozoic zircons of beach areas,

Relative zircon age probability distribution -Mesozoic
Mesozoic zircons are identified in the Riachuelos (~194-66.1 Ma, n = 9) and Palma Sola (~185.5-72.4 Ma, n = 9) beach sediments. The probable source of Mesozoic zircons in Riachuelos and Palma Sola beach areas is the Chiapas Massif Complex, because similar Mesozoic zircon ages from the Chiapas Massif Complex are also reported by previous studies (e.g., Weber et al. 2009;Cisneros de León et al. 2017).

Relative zircon age probability distribution -Cenozoic
Detrital zircon U-Pb dating results (Additional File 1: Supplementary Information 4 and 5) reveal that the Riachuelos and Palma Sola beach sediments are dominated by the Cenozoic zircons (~33.1-0.39 Ma and~64.9-0.4 Ma, respectively; Figs. 12, 17). The potential source for these Cenozoic zircons is the Eastern Alkaline Province, located along the coastal region of the SW Gulf of Mexico and formed along the Gulf-parallel extensional fault (Robin and Tournon 1978;Robin 1982). The Eastern Alkaline Province consists of alkaline volcanics dominated by basaltic andesites. The detrital zircon ages reported for the Eastern Alkaline Province vary from 65 Ma to 0.18 Ma (Cantagrel and Robin 1979;López-Infanzón 1991;Nelson and González-Caver 1992;Ferrari et al. 2005). Thus the Eastern Alkaline Province is supposed as the provenance for the Cenozoic zircons of the Riachuelos and Palma Sola beach sediments.

Conclusions
Zircon, apatite, magnetite, titano-magnetite, and ilmenite were identified in the Riachuelos and Palma Sola beach sediments. The zircon grain morphology and microtextures revealed that sediments were transported by littoral and aeolian currents and deposited in a highenergy subaqueous environment. In comparison with the Palma Sola beach sediments, Riachuelos beach sediments were lower in SiO 2 and higher in Sc, V, Co, and Ni contents, suggesting a higher proportion of detritus derived from intermediate igneous rocks. Statistical analysis revealed that the major, trace and rare earth elements were generally associated with terrigenous rather than biogenic materials. The chondrite-normalized REE patterns for the Riachuelos and Palma Sola beach sediments were enriched in LREE, with negative and positive Eu anomalies, indicating their felsic and intermediate igneous sources. A passive margin setting inferred from the tectonic discriminant function diagrams for the beach areas was consistent with the geological setting of the Gulf of Mexico.
Th/U ratios of zircon grains were > 0.1, indicating a magmatic origin for the Riachuelos and Palma Sola beach sediments. The Riachuelos and Palma Sola beach sediments were dominated by the Paleozoic and Cenozoic zircons, and Proterozoic zircons were limited. The Zacatecas and Nazas Formations in the Mesa Central Province were the most likely source for Paleozoic detrital zircons. The Cenozoic detrital zircons implied that the source terrane, which delivered sediments to the Riachuelos and Palma Sola beach areas, was the nearby Eastern Alkaline Province. Significant discrimination in zircon REE patterns with respect to age was not identified, which suggested that the zircon U-Pb geochronology is a better method to identify sediment provenance than zircon REE patterns and abundances. The resemblance in zircon surface microtextures and zircon age clusters between the Riachuelos and Palma Sola beach sediments demonstrated a similarity in their source rocks, transport distance from the source area, and wave energy.
Additional file 1 Supplementary Information 1: Quantitative compositional data of sand grains from the Riachuelos and Palma Sola beach sediments obtained by Scanning Electron Microscopy (SEM) with Energy Dispersive X-Ray Spectroscopy (EDS). Supplementary Information 2: Trace (Nb, Hf, Th, and U) and rare earth element data (in ppm) for the detrital zircon grains in the Riachuelos (Sample No. RIA4) beach sediments, Gulf of Mexico. Supplementary Information 3: Trace (Nb, Hf, Th, and U) and rare earth element data (in ppm) for the detrital zircon grains in the Palma Sola (Sample No. PAL20) beach sediments, Gulf of Mexico. Supplementary  Additional file 2 Supplementary Information 8: Post-ablation cathodoluminescence images of representative detrital zircons from the Riachuelos beach sediments. Supplementary Information 9: Post-ablation cathodoluminescence images of representative detrital zircons from the Palma Sola beach sediments.