134-7 Rock types used in the construction of the Alamo (Mission San Antonio de Valero) and a possible pathway for exterior surface deterioration

Session: Crossing Boundaries: Histories of Geoheritage

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Abstract:

Limestone blocks used in the construction of the Alamo (Mission San Antonio de Valero) across building eras derived from quarries within the Atco and Vinson Formations (Austin Chalk Group; ACG) in the San Antonio area. In Bexar County, the ACG forms an ~ 50 m thick section deposited near the southern extent of the Western Interior Seaway between ~80-90 Ma. As part of an on-going conservation project, we have conducted a stone survey to document the different lithologies used during construction spanning four periods (Mission: 1744, 1755-1759, 1762-1772; Military Outpost:1846-1850). We assessed rock type, weathering state, and other characteristics from approximately 200 individual stone blocks on the west façade (presenting face of the Alamo) and on a south façade at the SE corner of the church. As expected from the rock types comprising the Atco and Vinson Formations, both the west and south façades consist primarily of lime mudstone blocks, which comprise ~ 75% (west) to 90% (south) of the stones used during Mission era construction, with skeletal wackestone and skeletal packstone forming less abundant, but the most common, secondary modes. On the west façade, we documented a single grainstone and single tufa block during Mission era construction. Lime mudstone also dominates the Miliary Outpost era (~75%), but shows a greater diversity of stone types and abundances relative to the Mission era, including grainstone, tufa, and mud-chip conglomerate. Wackestone comprises the secondary mode in the Military Outpost era on the south façade, with a single example of packstone. During the survey, we documented ~10% (west) and ~20% (south) of deteriorating blocks with a granular/crystalline texture that also shows substantial flaking, cm-scale dissolution pockets, substantial iron staining, and locally developed black mineral clots. A distinctive lithology within the ACG contains early-formed pyrite (FeS₂), which is susceptible to oxidation during chemical weathering. A number of the deteriorating blocks show black mineral nodules that have a red streak when smeared indicative of an iron (oxy)hydroxide composition, indicating the oxidation of iron in the original pyrite and formation of sulfuric acid. We propose that deterioration of some building blocks follows a chemical weathering pathway focused on pyrite hydrolysis/oxidation, which forms the source of iron staining and the sulfuric acid capable of dissolving the primary limestone.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10301

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