Their mission: update geological knowledge of French Guiana by combining two complementary objects of study—the superficial regolith and the underlying bedrock.
The Guyanese challenge: geology in a tropical environment
“In French Guiana, studying geology comprehensively and in detail requires gathering information on the bedrock itself, but also on the substantial regolith that almost systematically covers it—the lateritic cover,” explains Jean-Yves Roig, geologist at BRGM and co-coordinator of the geological component of the GOYAVES project. Between the lack of roads or paths and the dense forest cover, access to the regolith is limited; and even the regolith must be removed to reach the parent rock. Acquiring geological information in French Guiana is therefore a real obstacle course.
Yet this is precisely the challenge of Work Package 1 (WP1): updating the geological knowledge of the coastal fringe of French Guiana, both for the regolith and for the underlying bedrock.
The regolith: a crucial layer
In tropical areas such as French Guiana, the bedrock undergoes intense weathering under the influence of the climate. Water gradually disintegrates the bedrock, first by fracturing it and then by creating a sandy layer in which the parent rock remains recognizable—the saprolite. Weathering continues, dissolving and altering residual minerals, which are eventually transformed into clays. These clays are themselves leached, leaving only insoluble elements—iron, aluminum, and manganese—which accumulate at the surface to form the characteristic red lateritic crust of equatorial landscapes.
Regolith is thus a succession of weathering layers, from parent rock to lateritic crust, often topped by a fragile latosol on which the forest grows. This geological structure is complex and varies according to the nature of the underlying rock, topography, and hydrography.
Why study the regolith?
Despite its importance, the regolith remains relatively poorly understood. Yet this layer concentrates much of the stakes related to human activity: it is the surface on which people live, build, maintain, exploit, and protect their environment.
It is also a potential source of resources. Besides metallic ores such as bauxite, iron, or gold, it provides major construction materials. There are also water resource issues: lateritic weathering begins with fracturing of the parent rock by infiltration water. When fracture density is high, this process may create hydrological reservoirs—precious in a region where exploitable aquifers are rare.
But a lateritic profile is also an unstable system. Landslides may occur, particularly on steep clay slopes saturated with water. These events are relatively frequent and sometimes large in scale, such as the recent landslides in the Roura–Kaw area in 2021. Detailed knowledge of the regolith, combined with precise topographic and rainfall data, is essential for preventing and anticipating such risks. Finally, erosion of lateritic covers is suspected to favor the transfer of particles rich in potentially toxic metalloids (mercury, lead, arsenic, etc.) into Guianese hydrosystems—an issue investigated in other research projects. All this concerns key issues for land-use planning and for understanding the interactions between humans and the subsoil.
An essential update of knowledge
However, the nature and properties of lateritic profiles are intrinsically linked to those of the bedrock.
Geological knowledge of the Guianese subsoil is largely contained in geological maps. The most recent one, a 1:500,000 synthesis map published in 2001 for the whole territory, relies heavily on airborne geophysics. However, this map lacks the detail needed for the GOYAVES project. More detailed work requires the 1:100,000 scale. But maps at this scale were produced between 1955 and 1962. Since then, geological concepts and study techniques—petrology, tectonics, isotopic geochemistry and dating, airborne geophysics, and high-resolution Digital Terrain Models (LIDAR)—have evolved considerably. Implementing all these techniques aims to update knowledge of the bedrock geology and therefore of the regolith.
The most suitable medium for synthesizing, visualizing, and using all this updated knowledge is a new geological map integrating both the deep bedrock and, of course, the lateritic cover. This updated geological map will be digital and linked to detailed databases.
Two weeks, two study areas, two approaches
The October mission unfolded in two stages. During the first week, the entire GOYAVES project team focused on the Saint-Laurent-du-Maroni area to explore, at specific sites, the various aspects of the project as a whole. A full day was dedicated to the “Paul Isnard” track. This forest road brings together, along its entire length, all the issues addressed in the GOYAVES project—historical, societal, and geological—concerning both regolith and underlying parent rock.
The second week was dedicated specifically to WP1 geologists. It took place along the coast, where rock outcrops are accessible—quarries, rocky beaches, cleared areas. Several days were spent near Cayenne to identify and characterize the different rocks present and begin constructing the geological history, a crucial basis: having complete data for well-studied areas allows extrapolation to less documented or less accessible zones.
Their work mainly involves field observation: examining rocks and their deformation, collecting samples for microscopic observation and further analysis. This meticulous work aims to identify, in the field, correlations between geological nature and geophysical signatures detected from the air. Once this correlation is known, it becomes easier to extrapolate the geology of the bedrock across the Guianese coast using airborne geophysical data. But caution is needed: geophysical responses are not always unambiguous—rocks of different types may show very similar signatures. Only the geologist’s eye, hammer in hand, can decide.
Preparing the field for young scientists
This October mission also had a significant logistical dimension.
Two PhD students were recruited during the summer to bring their expertise to the project. Their work will focus on a few carefully chosen target sites to acquire complete data from the parent rock to the lateritic crust. The Paul Isnard track is already one of these priority sites due to its relevance to GOYAVES issues.
The first postdoctoral researcher began fieldwork during these two weeks. He will focus on updating geological knowledge of the bedrock and the geological evolution of French Guiana. The second postdoctoral researcher focuses specifically on the regolith. A reconnaissance survey was conducted during the team’s excursion along the Paul Isnard track in preparation for more intensive campaigns scheduled to begin in January 2026.
Modern and open data
This update of geological data—of both bedrock and regolith—will underpin the entire GOYAVES project. Although relationships between Guianese societies and the subsoil have mostly been studied through the lens of gold mining, this is far from the only use. The Guianese subsoil harbors many other resources: critical raw materials, industrial rocks and minerals, water resources. It is also the source of natural phenomena and risks that must be anticipated.
This broader perspective will help project the effects of global changes—climatic, demographic, land-use—on the evolution of relationships between societies and the subsoil. Hence the importance of having a robust, harmonized, and accessible geological database.
At the end of the five-year project, all these new data and updated geological knowledge will be made available to scientific and political stakeholders involved in territorial development, through a digital geological map and dedicated databases. A collective resource to envision Guiana’s future through 2040, finally integrating the long-neglected underground dimension—too often reduced to gold alone. Knowing the subsoil means knowing the limits and possibilities of a territory: where to build, where to protect, where to exploit. Anticipate risks, preserve resources, protect the environment, plan sustainably.
Une connaissance du sous-sol guyanais à actualiser
© PEPR Sous-sol, bien commun
Une connaissance du sous-sol guyanais à actualiser
© PEPR Sous-sol, bien commun
Une connaissance du sous-sol guyanais à actualiser
© PEPR Sous-sol, bien commun
Une connaissance du sous-sol guyanais à actualiser
©PEPR Sous-sol, bien commun
Une connaissance du sous-sol guyanais à actualiser
© PEPR Sous-sol, bien commun