Introduction

Geodiversity ―which originally arose as an analogue to biodiversity― has become a more common concept in scientific literature since the 1990’s, generally linked to concepts such as geoheritage and geoconservation (Carcavilla et al. 2008). Although geodiversity has various interpretations, it is generally defined as: "the natural range (diversity) of geological (rocks, minerals, fossils, structures), geomorphological (landforms, topography, physical processes), soil and hydrological features. It includes their assemblages, structures, systems, and contributions to landscapes” (cf. Gray 2013). However, other definitions have been developed in order to highlight its relevance in the distribution of biotic communities and anthropic activities (e.g. Nieto 2001; Kozłowski 2004).

Geodiversity assessments can be developed through qualitative and quantitative approaches (Brilha et al. 2018; Zwoliński et al. 2018). The qualitative approach consists of descriptive-documentary methods to characterize the elements that compose the geodiversity; these descriptions can emphasize specific attributes as well as provide a general explanation of their values. The most common methods to carry out qualitative assessments are exhaustive bibliographical review, gathering experts’ opinions, fieldwork, and the elaboration of simple thematic maps. Furthermore, the qualitative approach does not integrate a numerical component, but it can support the elaboration of inventories of Places of Geological Interest (PGI).

Quantitative and semiquantitative approaches usually integrate a descriptive part and in addition implement analyses based on numerical parameters and indicators to represent, in a more objective way, the frequency and distribution of geodiversity elements in a particular area (Carcavilla Urquí 2006; Brilha et al. 2018). This has led to the further development and application of geodiversity indices or models using Geographic Information Systems (GIS) and map algebra. The results of these procedures can be complete geodiversity index maps or partial index maps (Brilha et al. 2018).

Qualitative and quantitative approaches can be developed simultaneously or separately and can lead to the recognition of sites with heritage value. When developed concurrently, these approaches have the potential to achieve greater relevance in territorial planning, as well as in the preparation of PGI or geosite inventories and the establishment of management strategies (e.g. Fragoso-Servón et al. 2014; Posada-Ayala 2015).

Regarding quantitative approximations, the most common method is through geodiversity indices (Zwoliński et al. 2018). In Latin America, the methodologies proposed by Serrano Cañadas and Ruiz Flaño (2007) and by Pereira et al. (2013) are usually implemented. Nonetheless, some other studies develop geodiversity indices through the adaptation, upgrading and modification of the aforementioned methods (e.g. Forte et al. 2018; Araujo and Pereira 2018; Silva et al. 2019; Gonçalves et al. 2020; Steinke 2021). Most of these methodologies apply GIS operations and map algebra, and three methods of analysis stand out for spatial quantification of attributes: (1) predefined natural regions (landscapes) and/or geomorphological compartments (e.g. Nascimento et al. 2018); (2) artificial units based on a grid (e.g. Pereira et al. 2013); and (3) artificial units based on centroids (e.g. Forte et al. 2018; Silva et al. 2019).

In Mexico, geodiversity studies, although still scarce, are increasing in number and have been performed within the scope of different disciplines such as geography, geoecology, geomorphology, and geology. Assessments with qualitative and semiquantitative approximations are more abundant and are adapted to the recognition of priority areas and localities for the conservation and dissemination of geodiversity (e.g. Posada Ayala et al. 2015; Canet et al. 2017; Canet 2023). Among these studies, those developed in different parts of Mexico by Zaragoza Álvarez (2014), Ramírez Sánchez et al. (2017), and García-Sánchez (2020) are distinct because they elaborated assessments of geodiversity through geoecology which, as a geographic science, takes physical-geographical landscapes as the basis for the spatial analysis.

In southeastern Mexico, Fragoso-Servón et al. (2014) and Pereira-Corona et al. (2015) developed quantitative geodiversity assessments based on geomorphological territorial units (relative relief). They included in their assessment natural features to measure the heterogeneity of the relief such as geology, hydrology, and edaphology. These studies helped in the evaluation of land use plans and to design zoning in natural protected areas considering a spatial congruence to protect geodiversity and biodiversity.

In northwestern Mexico, Posada-Ayala (2015), Herrera-Gutiérrez (2016), and Miros-Gómez (2017) implemented joint quantitative and qualitative approaches through the adaptation of geodiversity indices and field documentation of various PGI. These studies were also designed to contribute to the development of local management plans for natural areas and to propose activities and itineraries for geoconservation.

In addition, the studies of northwestern Mexico differ in that they have incorporated sites of cultural value as a variable in their proposed geodiversity indices. In these studies, the incorporation of cultural sites is justified because many of these sites reflect the traditional knowledge that local communities have about the location of certain natural abiotic elements within the territory. Therefore, the cultural variable serves the purpose of grouping natural geodiversity elements not identified in publications and medium-scale maps.

It must be noted that the recent geological evolution of the tectonically-active region of northwestern Mexico determined the distribution of biotic communities, and thus indirectly shaped the way of life of humans who settled in the region (inhabited ~ 10,000 years BP). Furthermore, the insular arrangement and climatic changes that occurred in the Baja California Peninsula and the Gulf of California led to a 'cultural freeze' during the Paleolithic that lasted until encounters with the European explorers (cf. Téllez-Duarte 2002). Therefore, the traditional knowledge of the territory that communities possess is something that must be considered in geodiversity studies.

The present study also takes place in northwestern Mexico, in an area of exceptional cultural importance due to the legacy of an indigenous community that keeps its worldview alive and have imbued the landscape with material and symbolic value. Therefore, this study has the premise of an integrative assessment of geodiversity ―through qualitative and quantitative approaches― in a region of outstanding geological, biological, and cultural importance. The documentary analysis and the adaptation of a geodiversity index aim to generate cartographic outputs for the identification of priority areas, which can then be used to support the conservation and the diffusion of the geodiversity of the study area. It is also expected that the results obtained will help to: (a) inform the decision-making processes of indigenous communities, (b) the elaboration of geoconservation strategies and future PGI or geosite inventories, and (c) the updating of management and land use plans for protected areas in the region.

Study Area

The study area is located within the Midriff Islands region of the Gulf of California; specifically, between the Isla Tiburón ―the largest island in Mexico― and the adjacent coast of the State of Sonora (Fig. 1). Isla Tiburón is separated from the mainland by the Canal del Infiernillo, a 40 km long strait whose width ranges from 10 to 1.5 km (Lancin 1985). Two smaller islands, Isla Dátil and Isla Cholludo are located to the SW of Isla Tiburón. The inland part includes the desert coastal plains and bajadas limited to the east by the Sierra Seri tectonic escarpment.

Fig. 1
figure 1

The study area (marked with a hillshade = 2,208.4 km2) was delimited by considering geomorphological aspects (drainage basin limits), sociocultural aspects (Comcaac territory), and updated cartographic information. Names of islands and some localities are in the Comcaac language (Cmiique iitom) with the Spanish names in parentheses

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The area largely overlaps with the Comcaac (also known as Seri) indigenous territory. The Comcaac territory includes Isla Tiburón and the Canal del Infiernillo, which is an exclusive fishing area for the Comcaac community. The Comcaac population is about 1,011 inhabitants (INEGI 2020) living in two villages: El Desemboque (Lat: 29.505051°; Lon: -112.394483°) and Punta Chueca (Lat: 29.014694°; Lon: -112.160266°), located in the northern and central part of the study area, respectively.

The southernmost part of the study area is outside of the Comcaac territory. The town of Bahía de Kino (Lat: 28.824579°; Lon: -111.937720°) is located in this part of the study area. This town has 8,000 inhabitants who are mainly occupied in fishing and tourism activities (INEGI 2020). Bahia de Kino is located next to a coastal lagoon known as Laguna La Cruz, where aquaculture and fishing activities take place.

The study area has an extraordinary biological importance recognized at a national and international level. Both the Canal del Infiernillo and Laguna La Cruz were declared Wetlands of International Importance by the Ramsar Convention (RIS 2009 and 2013, respectively). The islands and islets ―including Isla Tiburón― are part of the Gulf of California Islands Flora and Fauna Protection Area (APFF), established by presidential decree in 1978 (DOF 2001) and administered by the Mexican National Commission of Natural Protected Areas (CONANP). In addition, this natural protected area owns two international designations by UNESCO, as a Man and Biosphere Reserve (although this designation was withdrawn in 2020; UNESCO 2021) and as a World Heritage Site (UNESCO 2005), although it is included in the List of World Heritage in Danger (UNESCO 2019).

Materials and Methods

Qualitative Assessment

The cartographic information consulted and generated in this study, as well as the field work for confirmation, served for the descriptive analysis of the attributes of geodiversity. The descriptions are shown in the results.

Quantitative Assessment

The geodiversity index adapted for this study was previously implemented by Herrera-Gutiérrez (2016) in the arid region of the Colorado River Delta and surrounding areas; in turn, it is based on the methodologies proposed by Serrano Cañadas and Ruiz Flaño (2007) and Pereira et al. (2013). It establishes the use of cartography and geostatistical methods performed throughout a grid analysis that results in a geodiversity index map.

The index is developed in four specific steps: (1) definition of the scale of analysis, compilation, and elaboration of cartographic information; (2) selection of a grid resolution; (3) data analysis using GIS to generate the index map; and (4) field work to confirm the results.

Scale and Compilation of Cartographic Data

Due to the size of the study area, intermediate scale maps ranging from 1:50,000 to 1:250,000 were used to perform the quantitative assessment of geodiversity (Table 1). To avoid substantial biases on the information contained in the larger scale maps, the most appropriate level of legend information was considered to maintain a balance with the different information levels of each map.

Table 1 Cartographic datasets collected and elaborated for the analysis and mapping
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The cartographic data were obtained mainly from Mexican governmental agencies such as the Instituto Nacional de Estadística y Geografía (INEGI), the Servicio Geológico Mexicano (SGM), and the Simulador de Flujos de Agua de Cuencas Hidrográficas (SIATL). However, cartographic information for Isla Tiburón and some other coastal areas was not available, and thus the information was gathered in two ways: (A) digitalized from different recent studies such as thesis/dissertations, scientific papers, and reports carried out in the study area, or (B) generated with the analysis of Digital Elevation Models (DEM) and satellite imagery collected from international agencies (Table 1). In both cases the information was corrected and fine-tuned to maintain a similar scale of analysis and to reduce bias towards other cartographic information. In addition, field verification was conducted in the study area, which made it possible to clarify or rectify rock units and structures in specific sites.

The geological map presented in this study was prepared by updating and simplifying several maps published by various authors (Table 1). This map also includes the location of the paleontological sites considered in the assessment.

Furthermore, in this study a synthetic geomorphological unit map was prepared through photointerpretation of satellite images and morphometric analysis, as well as the consideration of previous cartographic work at different scales (Lancin 1985; Ortlieb 1991; Gutierrez-Anguamea et al. 2022). The synthetic mapping approach allowed for the identification of relief units that are classified based on their origin (morphogenesis). The classification was simple and did not include aspects such as the slope and the lithology for the nomenclature, since these factors were incorporated in the roughness coefficient and in the geological subindex, respectively.

Photointerpretation analysis was performed using ESRI satellite imagery. Additionally, a 15 m cell size DEM was used to generate morphometric cartography, which was integrated by a terrain relative relief map and a slope map. In this work, seven slope intervals were established adapting what was proposed by Durán Calderón et al. (2014) and the relative relief intervals followed what was proposed by Gutierrez-Anguamea et al. (2022).

The photointerpretation combined with the analysis of morphometric maps resulted in the identification of field breakpoints and the subsequent delimitation of relief units at a scale of 1:50,000. Since the study area is in a coastal region, the units located in the transition zone were also included in the mapping. Finally, the methodology proposed by Lugo-Hubp (1988) was adapted for the genetic classification.

Data processing and Analysis

Data processing was performed in ArcMap 10.5 following the grid counting methodology (Pereira et al. 2013). This methodology analyzes the geodiversity of a given area using a grid formed by square units. The grid pulls the numerical quantification of abiotic attributes present within each square from each thematic map or layer. Finally, the sum of the values obtained for each data set represents the partial index.

The determination of the grid cell size is still a matter of discussion (Hengl 2006; Pereira et al. 2013; Dias et al. 2021). In this work, the minimum mappable area for each map scale (see Priego et al. 2011) was used as a criterion to select the grid cell size. Knowing the different scales, it is possible to define the surface with the smallest area represented in each map and thus estimate and test the appropriate grid resolution that allows good differentiation of the elements. Therefore, a grid with 9495 squares of 0.5 ✕ 0.5 km was selected for the assessment (with an individual area of 0.25 km2 for each square).

Partial Indices and the Geodiversity Index

In contrast to the index proposed by Herrera-Gutierrez (2016), the methodological adaptation implemented in this work adheres to the following criteria: (A) Group within each cartographic theme (e.g. geology, geomorphology) the information concerning abiotic natural elements identified in previous mappings of sites of cultural value; (B) Avoid the use of small cartographic scales or regional scale maps (e.g. scales of 1:1000,000 or smaller); (C) Prepare detailed geomorphological cartography for the study area; (D) Reduce bias on the hydrography (see Miros-Gómez 2017); (E) Update the geological cartographic information; and (F) Exclude the use of the PGI layer as one of the variables that constitute the index.

Regarding the previous criteria, our adapted index is composed of four partial indices: (1) Geological index (IGeo); (2) Paleontological index (IPl); (3) Soil index (ISo); and (4) Geomorphological index (IGm). The sum of all partial indices results in the Geodiversity index (IGn), which is subsequently displayed on a map and classified into five categories. The index can be simplified in the following expression:

$$IGn=IGeo + IPl + ISo + IGm$$

The IGeo comprises 38 lithostratigraphic units, as well as major faults, remarkable folds, and ore deposits. This partial index was calculated by counting the units and elements occurring in each grid square.

The IPl accounts for the paleontological sites (specific localities and fossiliferous formations reported). It was calculated following the same procedure as the IGeo.

The ISo considers the groups of soils represented on the soil map elaborated by INEGI (2014). It is classified according to the World Reference Base for Soil Resources (WRB) of 2014. Each area or attribute on the soil map can contain an association of one to three soil groups. Therefore, this partial index was calculated by counting the soil groups occurring in each grid square.

The IGm comprises features that define and modify the relief (Table 1). This partial index also integrates the roughness coefficient according to the methodology of Serrano Cañadas and Ruiz Flaño (2007). In addition, the IGm also integrates the hydrographic network following the methodology proposed by Pereira et al. (2013) in their “hydrological sub-index”. However, to reduce any bias in the hydrological sub-index (see Miros-Gómez 2017), the streams of order 1 and 2 in the Strahler system were not considered. These streams were often imperceptible and did not significantly change the relief.

The IGm results from the sum of the following values: (A) The value obtained by counting the main geomorphological units, wetlands, bodies of water occurring in each grid square; (B) The value of the hydrological subindex within each square calculated as proposed by Pereira et al. (2013); (C) For the roughness coefficient within each grid square, the value of maximum coverage determined for each of the categories was considered (see Herrera-Gutiérrez 2016).

The sum of each of the partial indices results in the value of the IGn. With these values, an interpolation was performed using the Kriging method, which resulted in a Raster image that shows the distribution of geodiversity within the study area. To simplify the interpretation, the data were classified into five hierarchical groups (very high, high, medium, low, and very low) using the Natural Breaks method (Jenks 1967).

Results

Lithostratigraphic Diversity

The geological map, prepared with previous detailed maps from several authors (Table 1), aims to facilitate a clear appreciation of the heterogeneity of lithological features that make up the geodiversity of the study area. Thus, the information is presented in a simplified manner, maintaining the necessary detail for the analysis and, in turn, for the recognition of potential geoheritage sites or elements.

The qualitative analysis of the geological map accompanied by the documentary review of the geological and tectonic history supported the recognition of the geological events that had major implications in the geodiversity and orography configuration of the study area. Therefore, the local lithostratigraphic framework can be roughly divided into four groups that overlie the pre-Cenozoic substrate (also referred as basement; Oskin and Stock 2003a). The arrangement of the basement and each of these groups are linked with major events that are described below.

Pre-Cenozoic substrate is composed of units from the Paleozoic, Jurassic (?), and Late Cretaceous. The Paleozoic rocks are part of terranes accreted by the tectonic collision between Laurentia and Gondwana, during the assembling of Pangea (Campa and Coney 1983; Centeno-García 2017). Paleozoic units consist of highly deformed deep-water 'fossiliferous' sedimentary rocks (Poole et al. 1993), which were metamorphosed by the intrusion of Late Cretaceous igneous rocks. Indeed, these intrusions also promoted the uplift and erosion of the Paleozoic units, making them scarce and remaining as isolated outcrops known as roof pendants.

The igneous rocks are commonly granitoids emplaced during the subduction of the Farallon paleo-plate beneath the North American plate (~ 100–40 Ma). This event was responsible for the emplacement of large batholithic belts along northwest Mexico, and it is denominated as the Cretaceous–Eocene Mexican Magmatic Arc (Valencia-Moreno et al. 2021). Locally, granitoid outcrops are referred to as the "Sonoran coastal batholith" (Ramos-Velázquez et al. 2008). The basement has important implications for the heterogeneity of mountain ranges, which are composed of batholithic outcrops with isolated roof pendants arranged in two belts: (1) a 160-km-long eastern belt in coastal Sonora; and (2) a 70-km-long western belt composed of several plutons that crop out on Isla Tiburón and Cerro Tepopa (Ramos-Velázquez et al. 2008).

Group I corresponds to Oligocene–Middle Miocene basal sedimentary rocks that overlay the basement complex. Strata of this group comprise a variety of sedimentary deposits, which clast composition dominantly reflects the local basement substrate (Oskin and Stock 2003a; Bennett et al. 2013, 2016). Group I contributes to heterogeneity mainly over the eastern batholithic belt located in coastal Sonora (Sierra Seri) and on the NW part of Isla Tiburón (Sierra Alta).

Group II is composed of rocks related to the Miocene volcanic arc, an event produced by the subduction of the last segment of the Farallon paleo-plate, from Oligocene to Middle Miocene, contemporary with the broad Basin and Range extension that affected northwest Mexico. This event produced several stages of mainly felsic volcanism. However, prior to the cessation of subduction, the emplacement of intermediate volcanic rocks began to develop in restricted areas along what is today coastal Sonora and Baja California Peninsula (upper Comondú Group) (Oskin 2002; Ferrari et al. 2018). The volcanoclastic deposits and lava flows of this group are dominantly andesitic and spread throughout five tectono-stratigraphic domains limited by rift-related faults that are shown on Fig. 2 (cf. Oskin 2002; Oskin and Stock 2003a).

Fig. 2
figure 2

Geological map elaborated according to the objectives of this study. Developed from the previous work of Gastil and Krummenacher (1976 and 1977) and corrected with subsequent studies by other authors (see Table 1). In the legend the basement and the groups described in this study are shown with a dotted blue line

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Most of these domains are characterized by an upper volcaniclastic unit that deserves special attention. It consists of discontinuous outcrops of a welded rhyolitic ignimbrite known as Tuff of San Felipe. This tuff was originally described in the northeastern margin of Baja California Peninsula (Stock et al. 1999; Oskin 2002). Its distinctive age (~ 12.5 Ma) and widespread distribution make it a key structural-stratigraphic marker to reconstruct the subsequent development of the Pacific–North America plate boundary in the Gulf of California (Stock et al. 1999).

Rocks of group II increase the lithological heterogeneity of almost the entire study area. In coastal Sonora they overlay the batholithic belt, while on Isla Tiburón, large bodies of andesite and basalt flows are present over the central and southern part of the Sierra Menor and the Sierra Kunkaak (Fig. 2). However, the Sierra Kunkaak has not been mapped in detail yet; thus, the variety of geological elements has not been fully documented.

Subsequent events and groups are related to the opening and evolution of the Gulf of California oblique rift, which began in the Middle Miocene and can be divided into two stages: (1) a proto-Gulf stage from 12.5 to 6 Ma; and (2) a modern Gulf stage from ~ 6 Ma to present. The proto-Gulf stage is characterized by an early 'restricted' deformation that facilitated the localization of the continental rifting. The modern Gulf stage initiated when the Baja California Peninsula was 'transferred' to the Pacific plate and the plate boundary focused on the present Gulf of California (cf. Bennett 2009).

Group III is characterized by a sequence of pre-rift to mainly early syn-rift volcanic and non-marine sedimentary deposits originated during the proto-Gulf stage (cf. Oskin and Stock 2003a). Sedimentary rocks record the formation of rift basins filled with fluvial deposits. The two main rift basins in the study area are the Valle de Tecomate, which forms the central valley of Isla Tiburón, and the Canal del Infiernillo (Oskin and Stock 2003a).

The volcanic rocks of group III have a major influence on heterogeneity, mainly on western Isla Tiburón where rocks of group II are covered by units of varied composition of lava flows and breccias with ignimbrites to the top. Individual units of these ignimbrites are correlated to the northern Puertecitos Volcanic Province of the eastern part of the Baja California Peninsula (see Nagy et al. 1999). The age of these ignimbrites and the fact that they crop out in both sides of the present northern Gulf of California, make them important as tectonic markers to constrain the timing and among of displacement before and after the opening of the northern Gulf of California (Oskin 2002; Oskin and Stock 2003a).

Group IV comprises a continuation of group III syn-rift deposits with the addition of marine strata and a localization of the volcanism on the southern part of Isla Tiburón (cf. Oskin and Stock 2003a). These marine sediments and volcanic units correspond to the modern Gulf stage and they play an important role in the lithological heterogeneity of the southern part of Isla Tiburón. An area where extensive research has been conducted (e.g. Gastil et al. 1999; Bennett et al. 2015).

Particularly, the fossiliferous marine sediments interbedded with tuffs deposited on the southwestern basin of Isla Tiburón are highly relevant; these deposits fill in a paleo-embayment formed by right-lateral displacement on the La Cruz fault, and they record the first marine incursion event that occurred in the northern section of the Gulf of California about 6–4 Ma (Oskin and Stock 2003b; Bennett et al. 2015). Volcanic basin-capping deposits of group IV are also of high scientific relevance to understand the evolution of oblique rifting in this part of the Gulf of California, in addition to their relevance in shaping a variety of upstanding landforms of high scenic and didactic value (Fig. 3, photos d and e).

Fig. 3
figure 3

Significant landforms identified during geological and geomorphological mapping. a Hasteecöla mountain located near Bahía de Kino exhibits Early Miocene to Middle Miocene rocks; the peaks are composed of the Tuff of San Felipe. b Hast Yaxaxoj (Cerro Pelón) located to the north of Sierra Seri composed of a pegmatite outcrop that exhibit rock shelters. c Hastaacoj Cacösxaj (Pico Johnson) the highest point of the Sierra Seri. d Hast Hinamj (Punta Colorado) southern Isla Tiburón; the base is characterized by sea cliffs that expose Early-Middle Miocene basaltic flows underlying the Tuff of San Felipe and Late Middle Miocene conglomerates, and capped by a Pliocene dacite volcanic complex that forms the peak of Hast Hinamj. e Sea cliff of Hehe Hasoaaj Quih An Ihiip Iyat southern Isla Tiburón, it shows a spectacular view of a normal fault cutting Middle Miocene volcanic rocks. f Arches and coves located southeast of Isla Tiburón. g Marine terraces and coastal bluff located north of Bahía de Kino. h Playas located in the center Cerro Tepopa. i Anchored dunes of Hacac (San Nicolás). j Sand spit located inside the Canal del Infiernillo. k Bar composed of a dune ridge in Laguna La Cruz; note the sandbanks and channels eastward. l Tombolo of Zaaj Cheel (Estero Sargento) north of the Canal del Infiernillo. m Cuspate foreland of Punta San Miguel south of the Canal del Infiernillo. n Punta Víboras cuspate foreland with mangroves, channels, and sandbanks. o Estero Santa Rosa cuspate foreland with mangroves, marshes, and salt flats

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In summary, the geological map allowed us to determine how the lithostratigraphy and tectonics contribute to the geodiversity of the study area. From this perspective, the visual appreciation revealed a major heterogeneity over the mountain ranges and hills, while rift-related basins exhibited a homogeneous lithostratigraphy. However, the coastal margin along the south and west of Isla Tiburón is also heterogenous, which is related to the geologic events that occurred during the proto-Gulf formation and the opening of the Gulf of California. Furthermore, this heterogeneity in conjunction with the presence of an open, high-energy, deep-water coastline has promoted the development of a wide variety of landforms that are contributing to geodiversity from a geomorphological perspective. In addition, the qualitative analysis also highlighted some places and rock units that could integrate the geoheritage of this region, such as the ignimbrites whose value lies in the fact that they are regional tectono-stratigraphic markers, as well as the southwest marine basin of Isla Tiburón that records a marine incursion occurred in the northern Gulf of California.

Geomorphological Diversity

A total of 17 units or landforms were identified through the geomorphological mapping carried out in this work. This variety also nourishes the geodiversity of the study area, mainly in sectors where geomorphological processes are more active. The genetic classification of landforms is fixed into endogenous relief and exogenous relief; the latter is also classified into erosional and depositional reliefs.

The main endogenous relief units are the mountains and hills. Three mountain ranges stand out on the geomorphological map: (1) the inland batholithic belt that includes San Nicolás and Sierra Seri (Fig. 3a, b, c); (2) the Sierra Kunkaak, ranging from the south of the Isla Tiburón to Cerro Tepopa; and (3) the Sierra Menor and Sierra Alta located on the west coast of Isla Tiburón. The margins of the mountains are characterized by hills separated by a moderately dense hydrographic network and colluvial-alluvial troughs. Both mountains and hills usually have cultural manifestations; for example, some rock shelters located north of Sierra Seri (Fig. 3b).

Another endogenous relief unit corresponds to mesas, which are found only on the southern part of Isla Tiburón (Hipat Mesa). Their summits are distinguished by an extensive subhorizontal flat surface limited by steep slopes. Mesas consist of elevated outcrops of horizontal strata resistant to erosion such as lava or ignimbritic flows (Bennett et al. 2016).

Erosional exogenous reliefs include several types of landforms such as fluvial valleys, sea cliffs, coastal bluffs, and marine terraces. Fluvial valleys are characteristic of Isla Tiburón; they have elongated V-shaped morphologies surrounded by mountains and are usually carved by the action of fluvial currents that sometimes follow the lines of tectonic weakness (Durán Calderón et al. 2014).

Sea cliffs consist of escarpments along the coastal fringe, in many cases related to normal or strike-slip faults, as well as to rocks that are more resistant to wave erosion. They shape the south and west coasts of Isla Tiburón, as well as in the areas of San Nicolás and Cerro Tepopa (Fig. 3d, e). Sea cliffs are associated with abrasion platforms, headlands, and other landforms such as sea stakes, coastal arches, notches, and caves.

The coastal bluffs are steep shoreline slopes formed at the bottom of alluvial sedimentary deposits, placed 1–10 m above the high tide line. Coastal bluff formation is related to the origin of the bajadas, which were formed at a lower baselevel than the current one and, eventually, their primitive lower limits were abruptly cut into slopes of vertical erosion during the Holocene transgression (Lancin 1985 and references therein).

In many areas along the coastline, the coastal bluffs and dunes integrate the upper horizons of marine terraces characterized by exposed platforms with fossiliferous marine sedimentary deposits (Fig. 3g). Since this is a stable region with slow tectonic uplift, the formation of the marine terraces is associated with a eustatic variation in sea level that can be correlated with the IS 5e interglacial substage, which indicates the last time when the sea level reached a eustatically higher position than present was about 125 ka (Ortlieb 1991).

Depositional exogenous reliefs are separated into inland units and shoreline units. Two inland units are identified: (1) Bajadas, which are formed by generational coalescing of alluvial fans whose sedimentary source comes from the mountains and hills. They are developed over the two main rift-related basins in the study area; (2) Playas, that are flat arid basins that contain ephemeral bodies of waters, which disappear due to evaporation facilitating the formation of evaporite deposits (cf. Briere 2000). In the study area, the association of bajadas and playas can be seen near Cerro Tepopa, where a characteristic Bolson-type landscape was documented (Fig. 3h). However, near the coast both units can be associated with cuspate forelands, coastal lagoons, beaches, and coastal bluffs.

The shoreline units are diverse too. The sand dunes in the study area are arranged in dune ridges and dune fields commonly associated with beaches and playas. Dune ridges are generally aligned parallel to the beach, they can be fixed by vegetation or active that can move inland climbing on rocky slopes. Dune fields are large dune complexes with a combination of different types of dunes. In the study area the largest dune complexes are located north of Cerro Tepopa and in the area of San Nicolás (Fig. 3i). Coastal archaeological sites of symbolic value for the Comcaac community are common on the dunes.

Beaches are diverse, their shape is related to the direction and intensity of the approaching waves, as well as to the grain size of the sediment that is being deposited on the shore. The longest beaches are in high-energy areas exposed to the waves and are made up of coarse sands and gravel. Coves and pocket beaches are located mainly in areas sheltered by rocky headlands.

The sand spits constitute another unit formed by the successive deposition of littoral sand and shingle when waves meet the shoreline at an oblique angle. Their formation is associated with headlands that were part of the primitive coastline molded by the lower limits of the bajadas (Fig. 3j). The continuous deposition of sediment over time facilitated the growth of staggered spits with hook-shaped ends, eventually formed long and narrow bars that enclose bays and coastal lagoons. The bars can be partially flooded flat beaches or develop the presence of parallel dune ridges (Fig. 3k). When a bar connects the mainland with an island, another unit known as a tombolo is formed (Fig. 3l).

A cuspate foreland can be considered as a second order unit. It forms when longshore currents or waves are incident from two opposite directions, what facilitates the accretion and progradation of spits that can grow convergently until they merge into a triangular protrusion along the coastline (Lancin 1985; Uda et al. 2018). Cuspate forelands largely determine the geodiversity of the Canal del Infiernillo coast, in this area two types can be identified (Lancin 1985): (1) cuspate forelands formed by a single sand spit curved with a V-shape (Fig. 3m, n) and, (2) cuspate forelands formed by two converging sand spits (Fig. 3o).

In the study area, sand spits and cuspate forelands are characteristic landforms that influenced the formation of complex coastal systems such as lagoons (Laguna La Cruz) and the so-called 'esteros' (e.g. Estero Sargento), respectively. In turn, these systems have a variety of coastal geomorphological units that are included on the map, such as mudflats, tidal channels, and sandbanks, as well as salt marshes, mangroves, salt flats and pans.

Particularly, the mangroves situated in the study area are the northernmost ones in this part of the Pacific, together with those located on both margins of the Baja California Peninsula (cf. Glenn et al. 2006; González-Zamorano et al. 2011). These coastal wetlands are also associated with a wide variety of sites of tangible and intangible cultural heritage.

Lastly, the fan-delta-like units are gravel-rich alluvial fans that prograded from the adjacent mountains into the sea (cf. Nemec and Steel 1988; Whitfield and Elliott 2011). Fan-delta-like forms are obvious on the southwest of Isla Tiburón. Their arrangement is multiple, some composed of tributary and coalescing fans, while others are dominated by a single elongated stream. However, since the sedimentary analysis was not part of the objectives of this study, further studies are recommended to confirm and describe the fan-deltas in more detail.

Hydrology also plays an important role in the geomorphological configuration, mainly in areas of low slope. The hydrographic network has a varied structure with intermittent streams that are sometimes lost before emptying into the waters of the Gulf of California. Within the hydrographic network, the Strahler stream order is classified from 1 to 7. Streams of order 1 and 2 are associated with mountains and hills. Streams of order 3 to 7 are distributed along the bajadas and coastal plains, these streams can transport high volumes of water during the rainy season and generate significant changes in the relief.

In summary, the visual appreciation of our geomorphological unit map exposes that the most heterogeneous (i.e. with the highest diversity) areas are located along the coast ―a transition zone where geomorphic processes are highly active, facilitating the formation of a wide diversity of landforms― (Fig. 4). Although the mountains show some heterogeneity, the diversity of landforms apparently is not as high as in the case of the coastal zone. On the other hand, the rift-related basins composed of bajadas and playas are mostly homogeneous, this is because they are flat surfaces without abrupt changes where no other relevant landform was identified.

Fig. 4
figure 4

Geomorphological unit map. The photos in Fig. 3 are indicated on the map by white circles

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The variety of landforms recorded along the coast, together with their relevance as outdoor laboratories as well as their scenic and cultural significance, provides for students and all audiences a unique opportunity to disseminate and learn about the history of the Earth and carry out different recreational activities. Therefore, aside from contributing to a major geodiversity, these places of geomorphological significance can also be part of the geological and cultural heritage of the region. However, specific valuation studies will be necessary to confirm this.

Soil Diversity

The description of the soils of the study area was carried out considering the cartographic information by INEGI (2014). The cartographic soil units are represented by the association of up to three soil groups. For this description the soils corresponding to the first group are considered, since is the most dominant and comprises 60% or more of the soil groups.

Within the study area, 7 types of soils are identified. The most abundant are leptosols, usually formed on continuous rock or stony areas. This type of soil covers most of the study area and is mainly distributed over mountain ranges and hills.

Regosols are associated with mountain slopes and small bajadas; they are usually weakly developed soils with characteristics similar to the parent material. Arenosols are the product of the weathering of rocks rich in quartz and are distributed along the most extensive bajadas and coastal dune areas. Fluvisols are young soils that develop in alluvial deposits such as streams and valleys, they are mostly present on Tiburón Island.

On the other hand, Solonchak is a soil with a high concentration of soluble salts (at some time of the year). This type of soil is characteristic of coastal lagoons, estuaries and playas. Finally, calcisols and cambisols are the soils with the smallest and most dispersed distribution in the area, they are characterized by the presence of carbonates, clays and iron oxides.

Sites of Cultural importance as Input for the Geodiversity Assessment

The traditional knowledge of local communities represents an important input for the development of geodiversity studies. Particularly, the inclusion of traditional knowledge related to natural abiotic elements can help identify elements of geodiversity that cannot be easily identified during limited fieldwork or geospatial analyses in remote access areas. These natural elements are, in most cases, inherent to sites that have symbolic or material value for local communities.

Luque and Robles Torres (2006) previously conducted a study on Comcaac traditional knowledge, which resulted in an inventory with detailed mapping of sites of cultural importance within the study area. Cultural sites reflect the types of use or appropriation of the territory that have occurred over time, but they are also directly linked to natural elements of geodiversity. Some examples of this are: (1) sites for obtaining water were held in waterholes or 'aguajes', (2) ceremonies for the revelation of knowledge and acquisition of gifts were carried out in caves, rock shelters, and even pareidolic rock formations, (3) various types of rocks and minerals to produce tools were obtained from specific outcrops that are still present today due to the good state of conservation of the Comcaac Territory.

Although these natural sites were grouped within the cartography of each of the previously described geodiversity elements, it is also important to make this knowledge visible, as well as involve local communities in studies of this type, especially when working within indigenous territories.

Geodiversity Index Map

The results of the counting of each partial index and its distribution in the sampling grid are shown in Fig. 5. The sum of the values of all partial indices results in the geodiversity index (IGn) and the distribution of its values on the grid is shown in Fig. 5e.

Fig. 5
figure 5

Results of the counting of the partial indices and the geodiversity index: (a) Geological index (IGeo); (b) Geomorphological index (IGm); (c) Soil index (ISo); (d) Paleontological index (IPl); (e) Geodiversity index (IGn)

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The final IGn map for the study area is shown in Fig. 6. No transformation adjustment was performed on the data. Furthermore, no trend correction was applied.

Fig. 6
figure 6

a Geodiversity index map calculated for the study area. b The inset shows the histogram and trend analysis of the data used to perform the Kriging interpolation

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Very high and high values are found on the mountain slopes, mainly those located inland along the Sierra Seri, although they are also evident on the mountain slopes of Isla Tiburon especially around Sierra Kunkaak and Sierra Menor. Particularly, very high values are associated with the steepest mountainous areas. High and medium values cover some coastal areas associated with sea cliffs and wetlands; similar values are observed in areas of intersection between different types of soils and landforms, such as on the margins of playas (Fig. 5b and c). However, medium values cover mostly the hills of the entire study area. Low values seem to be distributed along the eastern coastal zone of Tiburon Island; these values seem to be scattered along the hydrographic network that cuts the bajadas. In addition, the very low values are distributed over the bajadas and playas, which are characterized by being gently sloping or flat reliefs without a wide variety of geological attributes.

Discussion

Qualitative vs. Quantitative Approach

In Mexico, geodiversity studies are regularly more qualitative or, at most, semi-quantitative oriented (e.g. Canet 2023). These studies generally trigger the recognition of elements of geological and geomorphological importance which, in turn, drives the development of inventories and valuations of PGI or geosites. The development of quantitative geodiversity studies continues to be scarce in Mexico. According to the experiences obtained in this study, this scarcity is related to the lack of unified and detailed cartography that can facilitate the development of geodiversity indices or other similar models.

However, the development of geodiversity indices should not be ruled out since they can be important in land management and planning processes because they display a visual synthesis of the geodiversity of a given area. In addition, they allow for the information to be communicated clearly to non-specialists and have a strong potential for the recognition of priority areas, especially at detailed scales. Particularly, the geodiversity index developed in this study was advantageous for the characteristics of the study area, which is a remote terrain with high complexity.

This work is an example where both approaches are developed concurrently and where they complement each other to carry out a detailed assessment, which is discussed in the following sections.

Geodiversity Values

The geodiversity index map allows for the association of high and medium IGn values with the variety of lithological and structural elements present on the mountains and hills of the study area. Likewise, elevation changes and terrain dissection increase the values considered within the roughness coefficient, which explains the presence of the very high IGn values. However, the individual analysis of the geological map reveals that the lithostratigraphic arrangement played a leading role in assigning high and medium values on the mountains and hills. This is notable mainly throughout Sierra Kunkaak, where there is no detailed mapping and lithological diversity was not reflected on the map, which is why the geodiversity index map shows low and medium values in that area.

The predominance of medium to high values located on the coastal margin are a separate issue. These values are mostly associated with the geomorphological diversity present in the coastal zone. The variety of geomorphological units that congregate in restricted areas within the coastal zone can explain the presence of these medium values. However, it must be considered that the geomorphological unit map elaborated in this study highlighted these attributes that are usually ignored in some geomorphological mappings. Therefore, it would not be possible to observe these values if a generalized or smaller scale map were used instead.

Occasional medium to high values identified on the margins of playas are associated with intersections between different groups of soils and geomorphological units. Although these transition zones exhibit important changes in relief, these values may be overestimated. A possible reason can be related to the detailed grid size used for the analysis, as observed by Lopes et al. (2023).

Low values are widely distributed along the hydrographic network that crosscut the bajadas deposits. The crosscuts generated by intermittent streams of order 3 to 7 of the Strahler system can produce changes in the relief and new landforms that can influence the geodiversity values. Previously, a bias towards hydrography was discussed by Herrera Gutiérrez (2016) and Miros-Gómez (2017) for their separate index map adaptation. According to them, this bias is not perceptible in a smaller scale analysis such as the one carried out originally by Pereira et al. (2013), but it could become noticeable in detailed mappings. However, here this bias is minimal since the currents of order 1 and 2 were not considered in the hydrological sub-index, because these currents do not generate significant changes in the local relief.

Finally, very low values are concentrated in flat areas with minimum slope, such as the bajadas or playas, where field observations corroborated the homogeneous geography of these areas.

Relation between Geodiversity and Geoheritage

Geodiversity and geoheritage are different concepts, but they can be correlated. In the case of our work, two scenarios were identified, one of non-correlation and the other of possible correlation. The first one corresponds to areas of very high to high geodiversity, mainly located along the mountain slopes. Although these areas remain in a good state of conservation and do have a scientific interest, they do not necessarily build a significant geoheritage, given their limited accessibility and undefined extension.

The second scenario includes the restricted coastal areas with mostly medium to high geodiversity values. This geodiversity is composed of rocky outcrops and extraordinary landforms with scientific interest and aesthetic value, and due to their easy accessibility, they have geotourism potential which would imply a management to ensure their sustainable use. Therefore, despite not showing the highest geodiversity values in the study area, this variety of physical elements can assemble a relevant geoheritage that must be subjected to a subsequent rigorous valuation. In addition, since they are located in the coastal zone, strategies must be established for their conservation due to their high vulnerability to unregulated tourism and housing development, as well as changes in sea level.

Geodiversity as a Management Asset

The results of the index map led to the identification of natural areas with high and medium geodiversity values that maintain a good state of conservation. Many of these natural areas record a part of the geological history related to the formation of the Gulf of California, as well as sea level changes that occurred during the Quaternary.

Among these areas are Cerro Tepopa, the coastal wetlands of the Canal del Infiernillo, and Laguna La Cruz, which stand out for their diverse geomorphology. These areas have also been recognized for their ecological and cultural importance, which is why they hold the designation as Wetlands of International Importance (Ramsar Convention 1994). Unlike other localities in the study area, wetlands show the direct relationship between geodiversity and biodiversity, where the physical environment facilitated the development of marshes and mangroves, which represent nutrient-rich areas that are important for refuge and nesting of various species of fauna (Glenn et al. 2006). In addition, wetlands have been provisioning sites for the subsistence of human groups over time.

The relevance of geodiversity has not been considered in the management and conservation efforts of these wetlands. Currently, local non-profit organizations in conjunction with the local communities and CONANP are working on updating the Ramsar designations and establishing management strategies for conservation of the Canal del Infiernillo and Laguna La Cruz wetlands (L. Meltzer, personal communication October 08, 2022). Therefore, this study is a tool with the potential to help in these projects establishing an accurate zoning that considers the attributes of geodiversity, as well as to promote its dissemination and conservation.

Another area located in the southernmost part of the study area is San Nicolás, a tourist center of regional importance due to the presence of climbing dunes and lee dunes of approximately 30 m in height. High impact activities such as sandboarding and dune driving are carried out in this tourist center (Fig. 3I). However, these activities do not contemplate the dissemination of geodiversity because there is not a widespread awareness of its relevance. In addition, these tourist activities trigger the deterioration of dunes, as has been documented in similar dune areas in Spain (Marrero-Rodríguez and Dóniz-Páez 2022). Thus, our study makes evident the need to implement alternatives that include the dissemination and conservation of these geological and geomorphological attributes. Here, geotourism can offer more sustainable alternatives than those currently developed.

The mountains and hills located near Bahia de Kino also have a high geodiversity value directly related to its lithology and geomorphology. Sites like Hasteecöla stand out from the landscape and have both scientific and didactic importance for understanding the geological history of the Gulf of California. However, sites like this exhibit a noticeable human impact because they are close to populated areas where housing and unregulated urban development threaten the conservation of geodiversity attributes. This problem is related to the fact that Bahía de Kino has no specific Urban Development Program to date.

Indeed, the Municipal Land Management Program (PMOT) of Hermosillo designates the Bahía de Kino area as a strategic territorial unit for sustainable urban growth, whose main objective is the promotion and diversification of tourist activities (PMOT 2018). In the face of it, our study points out areas and places of importance that should be included in the zoning of the future Urban Development Program for Bahía de Kino. In addition, the geodiversity of this area has a high potential for the promotion of geotourism through the creation of interpretative trails for the development of educational activities and appreciation of nature, activities that are increasingly attractive in this region and that can be promoted even during the winter, when conventional sun-and-beach tourism declines.

Sierra Seri is another area of interest according to the geodiversity index map; it displays relevant geology linked to a variety of sites of cultural value. Within the PMOT the entire Sierra Seri is considered as a conservation unit. The current state of conservation of Sierra Seri is good because it is within the Comcaac territory. Therefore, our study opens the opportunity to develop further studies to characterize its geological and cultural heritages, which could help the community to identify specific sites and activities to diversify their economy.

The areas of high and medium geodiversity on Isla Tiburón and on the neighboring islets also have an excellent state of conservation and exhibit outstanding geology of global relevance. The results of our study can contribute to the process of updating the management plans of the APFF and to the elaboration of a specific zoning for these islands. Moreover, this work highlights the importance of geodiversity in this coastal region and serves as a reference framework for subsequent processes of identification and valuation of its geoheritage.

The geodiversity index map developed in this study can support territorial planning, not only by identifying priority zones or sites, but also by identifying other activities to improve the management and use plans. However, these geodiversity studies should also be linked to the development of valuations and inventories of the geological and cultural heritage to attain more holistic strategies.

Conclusions

The elaboration of the geomorphological map and the integration of the geological map made in this study allowed us to document a series of localities of high scientific relevance, scenic value, and educational potential. This opens a window of opportunity for the development of geotourism activities aimed at raising awareness and dissemination of Earth Sciences. However, to achieve this it will be necessary to develop a well-defined and well-founded geoconservation strategy; as well as to convince the local authorities about the abiotic relevance of the study area.

The geodiversity index adapted for this study (IGn) showed results consistent with the observations made in the field and highlights areas of high to medium geodiversity that encompass various localities of biological and cultural importance. Therefore, the geodiversity index map represents a useful tool to be considered within the territorial ordering which is still a pending task on the central coast of Sonora. In addition, the geodiversity index can be used in ecosystem management processes through the establishment of more inclusive zoning within protected natural areas, considering that the attributes of geodiversity in the study area can control the distribution of biotic communities.

Quantitative studies of geodiversity have greater technical limitations compared to qualitative studies. The lack of cartographies and/or detailed mapping is the main cause of this limitation. However, quantitative studies are increasingly relevant since they generate information that can be analyzed by non-specialists, in addition to promoting the updating and development of local cartographies.

The qualitative and quantitative approaches developed in this study can serve as a reference framework for the development of other studies aimed at the characterization and valuation of the geological and cultural heritage of the study area.