|Fine-silty, mixed, Fluventic Hapludoll (El Batan) see pictures below|
Comments from Dr. Jamie Patton (Soil
Soil classification is a tricky business, particularly when you haven't actually seen the soil yourself and don't have complete lab analyses.
I am going to continue to go out on a limb (and dig myself a hole by going against the classification of OSU's expert) by still contending this soil is a Mollisol. (Fingers crossed we meet 50% BS).
Because the soil classification system is hierarchtical, we have to start at the basic levels of identifying our epipedon and subsurface horizons. Dr. Carter didn't mention anything about a mollic epipedon in his response, so that is why we differ greatly in our classifications. Mollisols come before Inceptisols and Entisols in the classification system, so if we decide we have a mollic epipedon then the lack of a strong B horizon is irrelevant.
From the data you sent back, I believe we do have a mollic epipedon. We meet the two main requirements: color requirement (<3/2 moist and <5/3 dry) and the depth requirement (= or > 25 cm with a little mixing of the AB). I am also assuming we meet the following mollic criteria: >50% BS, strong soil structure, and >0.6% OC. With a mollic epipedon, then the taxonomic hierarchy tells us we have a Mollisol. Mollisols come before Inceptisols and Entisols in the system, and therefore, the mollic epipedon "trumps" the classifying features of the other two orders (cambic B for Inceptisol and lack of B for an Entisol).
This soil then classifies out to a Hapludoll, because it has no striking features such as aquic properties, calcium carbonate , slickensides.... The great group can be a little tricky because of the lack of information. If we think the soil is saturated with water within 100 cm of the suface one month or more per year and 6 years out of 10, then it is an Oxyaquic Hapludoll. If that doesn't fit, and if there is at least 0.3% OC to a depth of 125 cm, then it is a Fluventic Hapludoll. If that doesn't fit, we classify into the "catch-all" Typic Hapludoll.
However, if we don't meet the BS requirement for a mollic epipedon, we have an umbric epipedon. An umbric epipedon with a weak B horizon (described next), would slip us into the Inceptisol order.
Because you seem to call a weak B in the AB and B horizon, I am going to assume you are describing a cambic B. A cambic B is described as a horizon with structure in at least 50% of its volume, a regular decrease in organic carbon with increasing depth, and the evidence of increasing clay content, a lighter chroma or removal of carbonates. A cambic B won't jump out at you as a definate B horizon and can sometimes be hard to distinguish from C materials, but with any formation of structure we usually assume at least a weak B. With a cambic B and an umbric epipedon, we would classify this soil as an Inceptisol. More specifically an Oxyaquic Dystrudept, if the soil is saturated with water within 100 cm of the suface one month or more per year and 6 years out of 10. If no signs of episaturation, then a Typic Dystrudept.
The family classification would go something like: fine-silty, mixed, surperactive, thermic (assuming mean annual soil temps are between 15 and 22 degrees C).... We now describe the activites of the clay when describing soils. If you are dealing with high CEC clays, then you call superactive, if lower CEC clays than active. I am assuming you are dealing with mostly smectites, so I went with superactive.
Here is a nice website that lets you follow along with the 8th edition keys if you want to see how I keyed this soil out.http://www.pedosphere.com/resources/sg_usa/orders.cfm
If you look at this website put out by the NRCS (http://www.nrcs.usda.gov/technical/worldsoils/mapindx/index.html), it shows maps of world soil orders, moisture regimes, and temps. With an approximate location of the soil near Mexico City, it looks like a Mollisol would work, but we might be in the ustic moisture regime. So, we would have a Haplustoll rather than a Hapludoll.