10-3 Unique Soils and Plant Endemism along the Geothermally altered San Juan de Los Planes Fault, Baja California Sur

Session: Geomorphological, Hydrologic, and Geothermal investigations of basin aquifers in Baja California Sur (BCS), Mexico (Posters)

Poster Booth No.: 40

Presenting Author:

Sula Vanderplank

Authors:

Vanderplank, Sula¹, Rebman, Jon P², Sanchez Romero, Abraham³, Lim, Carlos⁴, Medel Narvaez, Alfonso⁵, Bennett, Scott E.K.⁶

(1) Conserva Loreto, a program of The Ocean Foundation, Loreto, B.C.S., Mexico; Biología de la Conservación, Centro de Investigación Científica y Educación Superior de Ensenada, Ensenada, Baja California, Mexico, (2) San Diego Natural History Museum, San Diego, CA, USA, (3) Herbario HCIB, Centro de Investigaciones Biologicas del Noroeste, La Paz, Baja California Sur, Mexico, (4) Centro de Estudios Aplicados de Agua en Zonas Áridas (CEAAZA), La Paz, Baja California Sur, Mexico, (5) Herbario HCIB, Centro de Investigaciones Biologicas del Noroeste, La Paz, Baja California Sur, Mexico, (6) Portland State University, Portland, Oregon, USA,

Abstract:

Clay-rich grus deposits localized along the El Sargento segment of the San Juan de los Planes fault zone represent a previously unrecognized fault-controlled pedogenic system in the Cape region of Baja California Sur. This E-dipping normal fault zone separates Cretaceous granitic rocks to the west, from Quaternary alluvium in the San Juan de los Planes basin to the east. The fine-textured soils occur in a narrow (< 150 m-wide) area along the fault zone and are hypothesized to originate from hydrothermal alteration associated with recent but currently inactive hot spring activity along the fault, resulting in mineralogically altered parent material and pronounced shrink–swell behavior. The physical and mechanical properties of this substrate appear to inhibit the development of deep rooting systems through repeated expansion–contraction cycles, thereby constraining large woody vegetation establishment.

As a consequence, these fault-associated soils support open, herbaceous-dominated plant assemblages that differ sharply from surrounding xerophytic scrublands. Floristic surveys indicate that multiple narrowly endemic plant taxa are restricted to this substrate, suggesting long-term geological control over ecological isolation and local endemism. Among these are at least two plants found nowhere else –Marina ‘cacachilensis’ sp. nov, which is in the process of being described, and a new and undescribed species of Heliotropium sp. nov.  This fault-zone habitat is dominated by small low-growing native plants such as these two new taxa and Euphorbia polycarpa, Boerhavia xanti, Kallstroemia peinsularis and Krameria erecta as well as grasses such as Bouteloua brabata.

We propose recognition of this unique system as a discrete microhabitat formed by recent hydrothermal activity along an active fault and introduce the term “fault meadows” or “praderas de falla” to describe these hydrothermally derived, clay-rich meadow systems. Recognition of grus meadows highlights the role of fault-related alteration in generating fine-scale edaphic heterogeneity and biodiversity in arid tectonic landscapes and demonstrates the value of cross-disciplinary collaboration to better understand how Earth processes can control the distribution of floristic microhabitats.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3, 2026

doi: 10.1130/abs/2026CD-13026

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